It would be very difficult to outlaw lobbying completely without raising serious free-speech issues. That is not to say it is not a troublesome practice.
What free speech issues arise? Why is bribery not allowed under free speech? I
Ah, but what is bribery? Explicitly saying "I will pay you $whatever if you will vote for X" certainly is, and that
is illegal. But what about the wink, the nod, the agreement to hear someone talk, the big campaign donation, the implicit assumption of some form of
quid pro quo even if neither party says so aloud? This is where the problem really lies.