Okay, I wonder why they make so many irrelevant changes in the movies, like:
Why did they bother to cast Penelope Clearwater but they didn't have her petrified?
Whyn was the scene where Harry asks Lupin "Why didn't I get a chance at the boggart?" set on that stupid bridge they added instead of in Lupin's office? It would save them a whole bridge to build and by doing it they cut the gridylow and Snape giving Lupin his potion! :(
Why did they have Harry talk with Sirius in the fire before he found out about the dragons instead of afterwards?
Why does Narcissa have bron-blonde hair?
Why did the have Dumbledore tell Harry to hide under the tower instead of freexing him under the cloak?
Why was the quidditch match (the one where Ron thinks he's drank Felix) set in a heavy snowfall instead of in the sunshine?
Please respond. I have made a lot of topics but no one really responds to them. I'm sorry if I sound harsh, but I really would like to get more posts.

Okay, first things first. How much is changed in the movie depends on the director's vision. They have to think about the budget and the the time constrants. And they have to make sure it flows well and makes sense to non-readers. So, to save time and make it flow and make sense, they can't spend time on say for example, the Dobby and Winky storyline in Goblet of Fire. Also some of the more cosmetic things (Narcissa's hair, etc.) are just that, cosmetic and probably looked better on film. Nobody's going to see a movie that looks horrible, right? Now, on to your response problem. Sometimes it just takes time for people to respond. Don't get discouraged!

Okay, I'll take these one by one, but most of the reasons behind the changes are quite obvious. But you need to remember, the two are drastically different mediums and sometimes what worked on the page didn't translate to the screen unchanged.
- We do not know that in the original script Penelope Clearwater wasn't petrified. Her appearances in the film could easily point to the fact that her petrification was just, in fact, removed from the final theatrical release of the films. So in fact the actress, Gemma Padley, may have filmed scenes involving her attack which may even appear in the Ultimate Collector Editions of the films on DVD. That said, maybe she was kept in as a nod to the fans because her sub-plot was removed - and maybe as a safety net in case Percy ended up with her in a future book (remember, Chamber of Secrets was pre-Order of the Phoenix even).
- To make the film more visually compelling (something Alfonso Cuarón brought to the films, I believe) many scenes which were confined to indoor settings in the books could be transported into interesting settings. The bridge was one such location, something that needed to be introduced to link all the settings of the film to each other.
- Yet again, script to screen this could have been changed through editing and pacing, but most likely this is because the point of that scene in the book was to create much more tension in Harry and to slowly build up his fear and anxiety while awaiting the First Task, but in the film is used as a basis for mistrust of people on Hogwarts - such as Karkaroff.
- Narcissa Malfoy's hair colour in the novel tied her to her husband and son, and differentiated her from her sister, Bellatrix (quite literally portraying their different sides which was originally hidden). As Narcissa was introduced in the films two stories later she was not in the presence of either her son nor husband and as such having her be completely blonde wasn't required. Instead she is shown with her sister who we have to believe are related within seconds of their entrance and having similar hair colours tied the two easily together - while the blonde streak hints at her unwillingness to take to the dark side and her relation to Draco.
- In the novel Harry is frozen on the spot, but as we are being fed this story by an omniscient narrator we are able to be told the grief and complete feeling of helplessness of Harry at this point in time. On the other hand in the film we do not have this insight and all emotions have to be portrayed externally and having a frozen face hidden under a cloak doesn't tell us, as the audience, anything.
- In all the films there have been different Quidditch matches, all of which had a different weather set up. In that vein a different look at the sport is far more interesting to the viewer - who wants to see yet another sunny game? Dull. On a more in-depth level you have to take into account the overarching tone and look of the film, and Half-Blood Prince had a very dark and moody look juxtaposed by the romantic comedy going on with the characters. A sunny scene would have been out of place, while an icy setting fits both the films look and emotional place of the characters at that point in time - the tension between Ron & Hermione is building.
So, while I may have gone on and on, in the end I'm just saying that while I don't like any changes either - every single one would have been discussed extensively behind the scenes. Sometimes it's okay.
STALK: General Ironbeak

I agree with what others have stated above. Maybe with Penelope, they saw that as somewhat redundant. Nobody likes that. Plus, her almost randomly appearing ( I know she had that small cameo, but that was it) would be weird and confusing. Visually compelling is a good reason for the bridge scene. I think it looks better that way. As for the frozen in place, well, as an audience member having never read the book ( just saying if I were one of those), staring at a blank face would be, well, boring. Seeing his emotions is more like it.