With the things I know about Avast, I think that the Blocker prevents infectable files from doing the checked actions specified on this tab.
No, as Rejzor said, it's a kind of heuristic against unknown viruses. The selected actions are prevented always, no matter if the file performing them is infected or not. That's why it's called
behavior blocker - because it blocks suspicious behavior, not files.
In any case, it's quite an obsolete feature and I wouldn't recommend to use it much (as you found out yourself, many applications open files for writing even if they don't have to - I believe the Explorer in the first post opened the file for writing (+reading) just to access the file's properties).