A few questions in that direction:
1) What is the difference between lobbying and bribing?
2) Does one need something like a "lobbying license" or something like that do lobby?
3) The Money which lobbyists give to officials, is that documented somewhere or is it actually "under the table" without any official knowledge of it?
4) Why, even though everybody knows how lobbying affects American politics, does never anybody speak out against it? Why are there no major protests against lobbying?
In short: Why isn't lobbying illegal?
Because it's useful. We need lobbying because most politicians don't know anything about the subjects they're trying to regulate. We need lobbyists, unions, activists and all kind of pressure groups to tell them not to do this, or that, because it would hurt. That's democracy and justice. Many citizens, when they vote, are some kind of lobbyists, following the advices they heard on TV. The only system where there can't be any lobbying is anarchy