No, that's fine, and how things operated for years. Not sure how free lunch really helps attract more folks. However, that doesn't help with other infrastructure like the website, unless you want to ask organizers to pay out of their pocket.
You'd have to ask Miles (who commented above) why he made the switch to sponsorship.
I replied directly above, but the other reason sponsorship is attractive is because it's hard to host a group of 20-30 people for lunch usually.
We've lucked out in Boulder & Denver where we can make a reservation for that big, but a lot of places I've talked to want event fees, so I'm still going to avoid that, but it'd be nice to have the option.
You'd have to ask Miles (who commented above) why he made the switch to sponsorship.