Interestingly, I don't think it matters whether or not there has been an increase in renting, and if so, whether it is "commercial."
What matters is that owners in those systems
think there has been.
Surprisingly, Wyndham is not really in the business of owning inventory for rental purposes. I suspect that's partly because Wyndham's hotel brands are not exactly what I think of when I think "let's go on a nice vacation," and partly because of historical accident. The timeshare end of the business started as an independent entity (Fairfield) and that entity did not have a rental market presence. They were a straight-up real estate developer that was into planned "leisure communities" when they realized timeshare had a better ROI. They then bought Wyndham to give them some brand recognition. (I still think that was a mistake given the nature of that brand, but what do I know?) The hotel side and the timeshare side never really integrated beyond timeshare owners getting comped status in the hotel loyalty program, and some minor crossover for loyalty points. So, Wyndham doesn't really care* if someone is undercutting them in private rentals, because there is nothing in particular to undercut.
Marriott is a different ballgame. But, I suspect Marriott is not worried about "Joe's Luxury Resort Rentals" because Marriott is a trusted brand name, and who is this guy Joe? So I also suspect Marriott doesn't really care about Joe because he's not even the fly bothering the horse, he's the mite on the fly that's bothering the horse. Not worth the expense to track down and do anything about. What's more, Marriott is much (much!) bigger than the vacation club end of the business. I suspect there are dozens and dozens of cash rooms in the portfolio for every single timeshare unit. If that's true, it
can't ever be that important.
But, what happens when the sales floor starts seeing prospects balk with a stated reason of "There's never any availability for what I want! Why would I want more points?" It's especially interesting if those balking prospects say "because renting." It doesn't even matter if it's not actually "because renting" but just because some resorts are really popular and others aren't. By doing something "about renting" the developer can give the impression they are improving availability for owners. Wyndham's Owner Priority thing is easily the most obvious of these, but they all play a similar role.
DVC is a little different. Disney very much is in the rental business. Disney has l
ess than five cash rooms per DVC unit (and they've been busy moving the former to the latter). There is also a highly visible, consolidated, large-scale rental ecosystem that has a reasonable degree of trust among the target market. In short, Disney might be the timeshare operator with a hotel business that is most threatened by an existing rental market.
Now add in "I can't book what I want (because of those evil renters), why would I want more points?" and you might have a company with some motivation.
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*: There is one thing Wyndham cared very very much about: Owners using cheap resale points with the benefit of developer-tier room discounts and upgrades, because those discounts/upgrades came out of sales' marketing budget. They put a stop to that a few years ago.