I think you have the dichotomy wrong.
A registered agent is, among other things, a services mailbox. And a “shell” company, whatever that is, isn’t necessarily offshore.
Many US companies, even very ordinary ones, incorporate in Delaware for various, mostly good, reasons. IIRC even YC strongly recommends this. Unless the company actually has an office in Delaware, it will use the services of a registered agent in Delaware to satisfy the requirements of Delaware. Then the company will register to do business in whatever state it’s in. Or it could break the law and not register.
The only funny business here is that, at least traditionally, there is no requirement to inform Delaware of the beneficial ownership structure of the company. This seems to be changing — the US is pushing back against companies with anonymous ownership.
The big question, to me at least, is why anyone expects bad actors to fill out fancy new forms correctly.