Let's look from another perspective.
Let's say I'm an open bowler, i come in to bowl and I'm put next to a league. The league bowlers are not nice to me and my group (for reasons of lane courtesy, I'm drinking some beers, or I'm being too loud, etc). When that happens what makes me want to even consider joining a league? If I'm new to bowling, I would be turned off by that type of treatment. And yes maybe it was just that league, but my perception might be, well all leagues must be like that. Doesn't help our game grow any does it?
Like i said in my center the situation only comes up once or twice a season. I'm not going to tell someone hey you can't bowl tonight when I have lanes available to sell. League bowling is the backbone of our business, but if we had no open bowling we would never get any new blood into leagues. Bowling is more about friendships and community than the actual sport at a league level, how can we grow if we aren't more inviting to the open bowler? The open bowler comes in, gets hooked, and joins a league at least thats the hope. I don't know any centers that have people plainly come in the door and ask to join a league (a couple here or there but not many) our jobs as bowlers and proprietors is to get those open bowlers to have fun and want to bowl a lot more.