Wow. Serous question: Why would anyone do this for any reason? Makes no financial sense to me. I suppose the only thing I can think of is a perceived indirect 'marketing' effort that may be gained by a club pro using league success to promote the club name in the community...but then again, I say 'perceived', because tennis being the niche/subculture activity that it is, anyone who is already in this market knows the name, and is either 'buying' from them or not...
Adult leagues is a recreational, non-revenue generating activity for the participants...so paying people to play really sounds like some seriously misplaced sense of goal-setting imo...just weird. If I'm missing something I'd love to know it...
I can see some wealthy benefactor putting up money to create a weekly round-robin or some such thing to bring the game to new players, or subsidising people who genuinely cannot afford or do not have resources/infrastructure to play -- these are altruistic/charitable ventures...but to pay people to play on a adult league to beat a bunch of other people, at least half of whom participate as they would in a bowling or softball league (ie might be drunk, tired, purely social, whatever)...that's very odd to me...