That was just my impression on the three choice of actions, but right that might be a time where you can not join but later you might get the chance at some other point.
I didn't quite follow what you were proposing.
Why did you feel that they might not be joinable?EDIT: did you mean because none of the options was "join them" or "become their friend" etc.?
I think the faction experience would be cheapened in a way if you just randomly encounter the Stormcloaks and suddenly you are able to become a trusted member.
Also keep in mind Andrew Reiner just ran into them beside a river. It was Kevin Kelly who first heard rumors about the Stormcloaks in Riverwood, and he actually got a quest that said "Find and join the Stormcloak rebellion" - it seems pretty cut and dry if you have a quest that says you must join the Stormcloaks.