I remember people dropping them into the can, and every so often you'd hear about someone going to the ER because they accidentally swallowed one. That reason, and to cut down on litter, is why they switched to the tabs that stay on.
I just looked this up. The stay-on tabs are less hygienic as the tab goes down into the drink, so the liquid picks up whatever was on the lid of the can. The tabs that pulled off completely didn't have this issue. But apparently it doesn't bother anyone.
@Lostpoet The tabs that pulled off were phased out in the 1970s. If you were around in the 50s, you'd remember the cans with no tabs at all, that needed a "church key." The Revels had a song about this.