Yeah this bites me nearly every day. I can only assume that they have some data that this helps 60%-80% of their users in some way, and they are optimizing for the common case rather than the uncommon case.
Not sure that it's the right decision, even if it helps the common case, but it may be. And I say this as somebody who's not particularly fond of Google as a company or its policies.
It could also just be that they [incorrectly] assume their users are dumb, their algorithms are smart, and have forgotten what made them big in the first place. IIRC, besides PageRank, one of their original usability improvements was AND-ing query terms by default instead of OR-ing by default like their competition.
In some cases I think it would be useful to see a count of results with one of the terms removed, but there's no way it should override _my_ search terms. That just seems broken.
Not sure that it's the right decision, even if it helps the common case, but it may be. And I say this as somebody who's not particularly fond of Google as a company or its policies.