Can someone tell me what I am supposed to believe is allowing me to do this?
No.
Seriously, you are better off NOT trying to explain this one and accept that maybe for once it's necessary for gameplay to trump immersion.
And this is from a guy who has always fought on the side of immersion, including ship and module transfers. This is way beyond the ship transfer issue in that regard.
How you explain it is up to you. You want to keep your immersion going? Dock at a station first. Have your friend dock too.
But if docking became a requirement, then this feature would lose the vast majority of its potential users. How would explorers use it, once they're out of the bubble?
Here's a sample problem: You join a ship and go exploring, spending a productive session jumping towards Sagitarius A*.
Next day, you're online, but your captain isn't. What do you do?
Next week you decide you've had enough of this and want to go home, but the captain wants to continue to SagA*. What do you do?
Even if you have solutions for those, there are a dozen more you can come up with that will in some way hurt the ol' immersion.
This is something that I think has to be accepted and NOT explained. Explaining will only make things worse.