Maybe increased popularity to the legislator in question? Surely no bill comes without some ulterior motive.

Increased popularity isn't an ulterior (meaning hidden) motive, it's explicitly the foundation for democracy. The whole premise is that we exploit human selfishness for net good by setting up a system that (in theory) rewards making the largest number of people happy. There's nothing ulterior about that motive, it's the point.

Where things go wrong is when money becomes involved, because then the system rewards making the wealthiest people happy, not the largest number. That's an ulterior motive.

Why is buying votes by using government power/money good, and buying congress votes through lobbying bad?

I didn't say that buying votes with legislation was good, I said it's the premise of democracy. Whether democracy is good is an open question. For myself, I'm with Churchill:

> Many forms of Government have been tried, and will be tried in this world of sin and woe. No one pretends that democracy is perfect or all-wise. Indeed it has been said that democracy is the worst form of Government except for all those other forms that have been tried from time to time.

I don't think the premise of democracy is using money to influence politicans. If that's true then the rich have more power.

Please read my original comment. That's exactly what I said:

> Where things go wrong is when money becomes involved, because then the system rewards making the wealthiest people happy, not the largest number. That's an ulterior motive.

Of course one or more people benefit from every piece of legislation. If a legislator does it to get elected then that's the best case scenario for democracy, right?