I too am extremely frustrated with this feature... which seems to be Google created a folder ,,,,,,,,seen via IMAP that contains copies of all messages whether sent or rcvd. They state that this is good for you on some of their pages, When they rolled out gmail - it was like "You'll never have to delete another email message, ever". Well. what if I want to get rid of stale info, etc? Anyway, recently I went to gmail.com settings and found that the radio button /OPTION/ for not archiving is grayed out. See screen grab. So you can't turn it off if you want to. Further (and I tested this with one of my gmail accounts) if you DO persist in trashing the message in "Archive (All Mail)" folder -- IT GETS RID OF the messages in my INBOX and SENT mail. It's like they are "pre-archived". SO User BEWARE. I Think this is a Google Server issue, not an Apple Mail problem.
GOOGLE: DO NO EVIL! Please.
Choices are grayed out - not functional... WHY?