It implies that Patreon went the lazy route of "We don't want to bother with that" and just went with blanket restrictions or it implies that there's something else going on entirely.
You are forgetting one factor.
I agree that it's (relatively) easy to put in place, but to deploy such solutions you firstly need to know that you need them. And it's where lie the difference between Steam and Patreon.
Steam was launched by Valve, an AAA game studio. They knew beforehand that there's restrictions in what you can sell in that or this country, because they experienced it with their own games. Therefore, they added the regulation rules to their business model and platform right from the starts.
Plus, Valve used their own games to promote Steam, what mean that they experienced most of the possible legal issues firsthand. This helped them to slowly improve their rules and the way they enforce it.
And, of course, Valve is Valve. They revolutionized video gaming with Half life, then a second time with Counter Strike. Therefore, when Gabe Newell came asking for funds to revolutionize video gaming a third time, I assume that he had no real difficulties to get them. It can seem anecdotal, but it mean that right from the starts the company have enough money to attach the services of some of the best international lawyers.
At the opposite, Patreon is the dream of a relatively unknown musician,
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, who teamed with a developer so unknown that his Wikipedia page directly redirect to Patreon's one.
It's just a guess, but it's rational to assume that they had near to no clue regarding what their platform implied in terms of Law. They surely didn't known better than what they saw on internet. Therefore they believed that globally everything was permit as long as you ask if the person is major.
And like they were near to nobodies when they started, they anyway didn't had the funds to pay the service of a lawyer that would have known better than them. Nor to hire enough people to ensure a strong enough moderation.
So, it's not that Patreon was lazy or whatever else, just that they were totally ignorant of the effective mess they had to deal with ; "we are hosting porn content, like Tumblr, PornHub, and so many other sites, are doing, they are fine, so we are fine too". When you base your business on this kind of thoughts, paying dozen of thousands to a lawyer, just to ask him if you're right, is something that rarely cross your mind.
And all this was fine until someone told the world, or more precisely the payment processors, that there were content that shouldn't be available. But as we all seen, the platform was already reliable enough, at least in terms of potential, for the said payment processors to ask for a better regulation instead of cutting off all supports.