It is illegal for a lobbyist to bribe or attempt to bribe a congressman or any fed official, for that matter. So in order for it to be illegal there must be a quid pro quo (or offer) transaction. Nor can a member of congress accept a bribe.

So that takes care of the legal aspect of being a lobbyist. In fact, lobbying is a protected right enumerated in the constitution (petitioning congress). Most of us only like the lobbyists who agree with us and hate the other fellows' lobbyist as a threat to democracy or civilization or whatever.

Now explain how lobbying is unethical.