Some people cheat because they're not getting an education, what they're getting is a degree.
If someone has all the skills necessary to perform a given job, but an employer will not even finish reading their resume because they don't have a BA, I see no reason not to cheat in order to expedite the degree-earning process.
If you want to work with computers for a living, and you have a political science paper due in one day, what is going to be the better use of your time in the long-term, an all-night hack session, or actually doing your politics paper?
Also, I've never cheated, and I'm theorizing, but I think that people bright enough to carry out an A-worthy cheat could get the A if they did the work, and people that can't will either get a B or get caught. So in the end, it might be sort of fair anyway.