I run a Patreon to support a webcomic that I otherwise release for free - backers just get their pages a few days early. Also, backers get access to my little sideproject-comics as well - but I sell those comics via Gumroad too, for non-backers. And, occasionally, post them for free on Tumblr.
My rule of thumb is to give backers optional extras, but still keep most of my art public. Backers get to see sketches and WIPs, but most of my finished art goes up on Tumblr.
It's not Patreon's fault - how one runs one's Patreon campaign is entirely up to the individual. Some people just use Patreon as a tip-jar, and keep no content locked away behind a paywall - and that's fine too.
But remember - art is just as much art, even if you have to pay for it. Just because people are asking for money in return for art doesn't mean their art is of less artistic value than those who do it for free. Some people are using Patreon to supplement a very meagre income, and they deserve to be paid for their time and effort. If you don't want to pay them, that's fine - but you don't get to complain about how they choose to share their art.