If I was the gift-sender in that situation, I probably would have been touched a little that the bride cared enough, and was polite enough to send a thank-you note even though she thought all I'd gotten them was trash bags.
If I was the bride in that situation, I probably would have opened the box out of sheer confusion. I *do* try to show appreciation for gifts, no matter how odd or seemingly impersonal they are, but opening a box of trash bags with a calm, collected face is simply a level of etiquette I have yet to reach. I'd probably have opened the box wondering if they were particular trash bags that the gift-sender was really fond of, or if I was just confused on what was considered a wedding gift.
For some reason, while I wouldn't be offended to receive anything, of course, but I may have two thoughts. Either they walked through their house trying to find something to wrap up and send and all they found unopened was trash bags, or it was a PA way of insulting my cleaning. (I'd probably lean toward the latter, only because my cleaning isn't the best.) In that way, it kind of reminds me of how buying soaps and deodorants are a really risky present, because it's easy to read into a "hint" you may not have put there. (Or maybe that's just my social circle where it's considered a bad idea?)
While I do think the bride should have opened the box to check, kudos to her for having enough sense to still send a thank-you note for what she thought was possibly the strangest gift ever. I think a lot of brides may have decided to not send a note in face of what the present was.