Why would they bother? They make all their money on the ink cartridges. If this drives more cartridge sales it’s a win/win for them. They generally don’t make much, if anything, on consumer ink jet printers themselves
This printer will use refilled cartridges. It could even use cartridges that have been sent out under the 'instant ink' subscription scheme. People in that scheme usually end up with a drawer full of spare cartridges, as they send them to you like they are worth the pennies that they actually cost.
People willing to use refilled cartridges will use them regardless of the printer they’re going in, so again it’s no lost sale for HP.
Heck, it may actually save them money. At one point they were losing money on every printer sale and making it back in ink No idea if that’s still the case.
It’s negligible revenue and carries some risk. Say the printer works well, and these enthusiasts drive a couple of thousand more cartridge sales. And that encourages someone to develop an alternative cartridge, which is mechanically and electrically compatible with HP’s.
It just seems easier to fire off a cease and desist than to take a risk of other people messing around with your ecosystem.
Many corporate and government types, especially lawyers, want control. They will go broke trying to control everyone and everything.