Unless the provider is a POC themself, its racism plain and simple.
Everyone is entitled to see only who they wish, so no one can tell a provider they have to see black men if they don't want to, but their preferences have a racist background to it by prejudice and social conditioning, and it's important everyone take the time to reflect on that.
By and large, white men are the ones who rob, assault, rape, pressure for unsafe services, are unhygienic, etc. That's also probably because they make up the majority of clients, because they're also the ones with the majority of wealth in society. But anyone who says they don't want to see black clients because they had a bad experience with them, or some other similar reason, while probably not lying, is generalizing. I've had a bad experience with a person of basically every race, but that doesn't stop me from not seeing them again because a) every person is an individual, and one person's actions do not reflect others, and b) some SPs can get away with generalizing like this with black men because they're a minority, and they know it won't effect their business as much. If they said the same about white men, they'd have no clients at all.
Also the large penis thing? C'mon people, are we also going to claim all Asian men have small dicks? A person's skin colour doesn't reflect on penis size. Usually I've found tall skinny dudes or slightly chubby ones of every race always have the largest dicks, if we're being honest here
At the end of the day, every reason they have comes down to racist conditioning, and it's important people work to unpack this stuff and understand where it comes from. And yes I know people get their panties in a bunch when the words racism is thrown around, but instead of getting defensive, it's good to actually stop and think on the prejudices we have embedded in us, and why we think it's okay and acceptable to have them at all.
And absolutely providers should only see who they wish and are comfortable with, no arguments there since it's their body.