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.
But this is it, it's bribery just under a different name. There's no shady meetings in carparks with exchanges of cash. These people all do it the legal way and are not stupid enough to explicitly state what they want in return, but when they make that campaign donation they sure as hell will want their money back at a later date. It's a you scratch my back I'll scratch yours situation.
http://www.youtube.com/watch?v=seDYqUFvYp0The above video is from The Young Turks where they explain how Hilary Clinton was paid nearly half a million dollars for two short talks at Goldman Sachs. They insinuate this is essentially a bribe and if she gets in to power they'll be wanting a return on their investment.