Uh no, as defined by Reg T, brokers are free to let cash accounts immediately buy with unsettled funds. It's only a violation if you sell a security before the funds used to purchase it settle.

Margin accounts have a different legal definition and regulations.

Robin Hood is actually pretty unique in that its cash accounts are always restricted to settled funds; instead they decided to optimize for margin accounts that don't allow for increased leverage. Which is not something other brokers optimize for that I've seen.