I'm not sure the comparison to the phone company is very applicable. This would be the case if Patreon would be a pure payments proessor, like Mastercard or Visa (or maybe Stripe). But Patreon provides pages on their own website within their own brand identity for these creators, stores their digital media, promotes them through their search interface etc.
In my opinion, Patreon is more akin to a newspaper deciding which ads to show next to their articles or to a shopping mall deciding which shops to rent space to. And of course every newspaper will reject ads that would infuriate their readers (or journalists) and shopping malls will not rent to businesses that might damage their reputation (depending on how "upscale" it is this might be everything from soup kitchens over gambling outfits and payday lenders to brothels).
Now, you might disagree with them whether they have drawn the right line there and consequently take your business elsewhere (just as you might not want to visit a shopping mall with the wrong mix of shops), but I find it a bit of a stretch to frame this as a "free speech" issue.