Lobbying should not be legal. It is buying power and buying your way on something in return for a large contribution usually. It is not right to buy political favors. All should be voted on in a democratic government but that is far from the case when lobbying is in the picture. But, since lobbying is a way for big businesses and those with a lot of money to buy what they want within the government, they go for it, the legal way, rather than bribery.
But, in essence, it is the same thing. Should it be legal? No way! It is not the democratic way and people get things passed and influence government for their own selfish and greedy reasons so of course it should not be allowed.