It's called dumping and depending on the specifics and jurisdiction, it can be illegal. It falls under the general umbrella of anti-competitive actions.
In that case, almost every VC backed startup would be illegal.
According to the ftc for predatory pricing to be illegal you need to use it to drive out the competitors and then gouge customers until you recoup your losses.
In the US anti-trust is absurdly focused on whether there is costs in the form of high prices passed onto the consumer.