Even with a user agreement I think they need to gate their service behind that agreement, otherwise the agreement is optional and open ports are open for use.

You still need to form a valid contract - no notice, no assent, no contract.

If there's a gate that's being bypassed then this all changes but it doesn't seem like there is - and it doesn't seem like they can add one without breaking existing printers.

In this case, you're signing up for an account and entering into the user agreement to do so. There's a whole separate legal discussion to be had about the whole "scroll past it all and just click accept to make it to away" thing...