"Here's a sports car for you if you vote the way I like on this issue": this suggests that the sports car bought the vote, and so is indicative of corruption.
"I like your position on this issue, here's some money for your campaign to help you get elected, and that can only be spent for that": this suggests that the position is the cause of the money, not the other way around, and so is not indicative of corruption.
So when I call my elected leaders to voice up about an issue, that's bribery? Because what I'm doing is lobbying. Or, if I call my elected leader and patch in a friend in another state who's more knowledgeable about the domain, is that lobbying?
If it's not, then what if my friend and I both patch in an out-of-district friend? What if we form a formal club? If we want (a) freedom of political speech and (b) freedom of political assembly, it gets really hard to separate a grassroots interest group from a destructive commercial one. (And should civic interests always trump commercial interests? That's a political question, too!)
TL; DR Democracy can't exist without lobbying. With respect to paid lobbying, I might agree, but the devil's in the details.