Just so there's no misunderstanding, you wouldn't be the one asking someone to move, the airline would and they'd most likely do it on check-in or before boarding. Also if they didn't have seats available, they'd likely ask for people that don't mind or find a single traveler next to an empty seat and they would likely find people that didn't pay for seat selection either so it's not quite as bad as it sounds. Also, I believe that they check this when they assign the seats in the first place so probably rarely an issue.
Just browsing through my emails, since 2010, I have 18 confirmation emails for flights with AllegiantAir and I've never paid for seat assignment (I thought I might have done it once but none of my emails show paid seat selection) and I don't believe we've ever had to ask to have our seats moved (our kids are now 11 and 7).