Doing multiple mag dumps at a pistol range seems like perfectly legitimate practice to me. Quantity is important for pistol practice. When I'm shooting handguns it isn't a big deal to shoot 500 rounds at the range that day. With a rifle, I rarely exceed 100 rounds.
Shooters will certainly tear up the targets faster on a pistol range than a rifle range. I can understand why you are frustrated if you are the one that is putting in the work to replace the targets. The best solution is to get the pistol range shooters more involved in time and/or $$ of replacing the targets. If other stuff is getting damaged on the range because shooters are just spraying bullets without hitting targets then that is a whole separate issue to address.
