Where in the Constitution is the "permitted bribery" clause? Lobbyists are not permitted to bribe, at least not directly. They are allowed to arrange donations to political parties and PACs, as are any other citizens. Unions, non-profits, and individuals also make or arrange donations to influence the political process. This is much different than bribery, which implies a specific payment for a specific action. And while I agree that the politicians can be swayed to provide benefits to favored constituents, that works both for and against the corporations...
It doesn't really matter whether it's straight up bribery, or other forms of influence. As long as congress is in the business of deciding winners and losers in the economy, instead of letting the market do it, they will be the target of ambitious people looking to influence them.