I have had the same thing happen to me. Machines I wanted to play that someone else was playing at minimum bet. And yes, it has PO'd me.
However, they had every right to play that way if the casino had no problem with it. Maybe that's all they had. Maybe that's all they had and they wanted a little relaxation. 5 hrs of fun on $5, can't beat that. I know that at Shreveport/Bossier City LA casinos I have seen older people come for the day with only $20 and they had more fun than I did with 100x that.
Just have to put it in perspective.