And besides, if a politician passed a bad law thinking he did a great thing for society because corporate marketing was very convincing, does that make a difference?
As to your last point: what is a "bad law?" Does anybody really have any idea what works and what doesn't? Did the repeal of Glass-Steagall cause the 2008 crash? We really have no idea. Did NAFTA leave consumers better off? Who knows? Do minimum wages increase unemployment? Experts disagree. It's effectively impossible to determine what impact laws really have, especially on the economy. Laws and policies aren't judged as "good" or "bad" except in history books. They're judged based on whether they fit into ideological frameworks and prevailing narratives. And voters are part of that. They want politicians who will vote for laws that fit their preferred narrative.
It's almost never going to be the case that a lobbyist will convince a politician to support a measure they wouldn't otherwise support. Politicians all have agendas and preconceived notions. The most important thing lobbyists do is help politicians filter through he universe of proposals and convince them that particular ones fit within their preconceived worldview and platform.
I think many people have a hard time even conceiving that politicians could support big companies without getting some sort of kickback or campaign contribution for doing so. But a politician's first, second, and third top priorities are jobs. Big companies control the jobs, and for better or worse what's bad for big companies is often, at least in the short term, bad for jobs.