Earlier quoted context omitted.
So the banner must have been placed there after the server was seized. Keeping the server running long enough to gather evidence of the actions of their high-value targets was probably a higher priority than sweeping-up as many customers as possible.
Possibly there are laws related to entrapment? Allowing new user signups on a police controlled website might cross the line, where allowing existing users to finish their deals might not?
Entrapment requires the cops to push someone into doing something they wouldn't otherwise have done.
Cops buying drugs from a dealer isn't entrapment, but repeatedly begging someone to sell you some drugs after they initially say no would be.