Is it legal for a private company to impose a financial penalty on you without even explaining the reason?
The fines haven't been decided upon from what I understand, and you could fight them once they've been announced, Stripe is simply freezing their assets to cover the potential fine.
I'm sure the card networks can do what they want and it'd be months before a court could catch up though.
A judgment in 2015 clarified the test of a penalty clause:
"[T]he true test is whether the impugned provision is a secondary obligation which imposes a detriment on the contract-breaker out of all proportion to any legitimate interest of the innocent party in the enforcement of the primary obligation. The innocent party can have no proper interest in simply punishing the defaulter. His interest is in performance or in some appropriate alternative to performance."
via https://www.pinsentmasons.com/out-law/guides/practical-impli...
The problem in this case would be that Stripe passes on the liability for the penalties to its customers if those customers are breaking the contracts. Obviously a problem for the customer, but also not unreasonable from Stripe's perspective. If you're setting up a marketplace and want to act as a payment-provider for third parties, you're creating a very different liability environment than if you're setting up an account to sell your own products. If you start with a seller-account and then transition your product into a marketplace, someone will have to eat that additional risk, and I can't see a good argument for why Stripe would be the one.