Why do people say be proved???

As either form is acceptable, it just comes down to a matter of personal preference. No need to get whipped up about it.

Proved and proven are both acceptable.

(By the way, you're examples with "saw" and "gave" aren't really applicable in this situation. They would only be analogous if people were using the word "prove" in the past tense. e.g. The attorney never prove his case.)

EDIT: You know, I actually agree that proven sounds better. It just didn't sound like you knew that "proved" was technically correct.

Because either one can be used properly, depending on the context of the sentence.

A better question is why people think that the use of astericks (*) as bracketing devices for the purpose of adding emphasis is acceptable and grammatically correct when it is, in fact, neither.

I've heard more people say "I never saw it," than "be proved," but either way it makes me want to scream with discomfort.

Actually, "proved" is an acceptable alternate form of "proven." Check your dictionary.

The other two examples you give ("have saw," "have gave") are incorrect.

My personal pet peeve is "have drank."