Picard states in the same episode that Starfleet believes their procedure is better than accidentally running into each other in space.
We prefer meeting like this, rather than a random confrontation in deep space.
The use of "confrontation" is the key: deep space encounters with unknown species tend to be fraught with misunderstanding and mistrust. Imagine if it also happens to be the first alien contact you ever make!
Picard admits that it's a controversial practice, but seems to insist that it's better than the alternative.
Centuries ago, a disastrous contact with the Klingon Empire led to decades of war. It was decided then we would do surveillance before making contact. It was a controversial decision. I believe it prevented more problems than it created.
(Putting two and two together, I wonder if the disastrous first contact with Klingons was originally supposed to be in deep space? This episode was written before their first contact was depicted in Star Trek: Enterprise.)
This is also speculative, but I wonder if Federation first contact procedures are influenced by the first contact between two of its founding species. Humans invented the warp drive and made its first alien contact on the same day, as seen in First Contact. Also in that instance, the Vulcans chose to land and make contact with Zephron Cochran's science team rather than political authorities.
As to whether the Federation is "imposing" themselves, it's worth noting:
a. re-reading the script, Picard never actually offers/suggests Federation membership. He only offers friendship and mutual cooperation.
b. the episode establishes that the procedure works quite well: if the aliens feel threatened by the Federation and ask them to leave, they will.