PAS is not mandatory anywhere in the US unless it is on a Class 3 (up to 28mph) e bike in California.
As far as using a throttle goes it seems they expect you will not be pedaling which while possible is not the case for every user.
Yes a stand alone throttle system is entirely possible as well as a stand alone PAS system which is mandatory in the EU where the bulk of the new breed of e bikes is coming from or the Asian manufacturers are trying to emulate.
There is a little more to it than an on off switch and two wires which is explained by others if you google e bike throttle perhaps.