They might have some automated system that does it. With the amount of money going around and the ease of deploying such a technology (single digit false positives don't really matter to the modern megacorp) I'd somewhat expect it to be standard right now.
The really gross but unsurprising part is their sheer unwillingness to talk about this instance and actually work with the customer to clear things up.
It's possible that the van sale was what triggered the fraud team to take a closer look at the account. I don't know if Stripe bans weed sales but if they've been selling normal quantities I doubt the algorithms would have them pop up as suspicious.
Then a huge sale goes through (company van) and an actual human of the fraud team looks into the normal products sold through the account. If they then find that the products sold violate the terms and conditions, they've got a reason to lock the account.
They don't have a good reason to block the sale of the van, though, especially if their ex customer has provided the necessary paperwork. They can lock future transactions and even all pending transactions if they have to (and demand proof that they don't violate the TOC for them to go through), but after filing all that paperwork the $3300 transaction should just go through.
Stop victim blaming and throat goating Stripe, it's disgusting.