Doping, which pretty much renders your entire career as fraudulent.
If I was a 140 ranked tennis player (roughly) who sometimes made it into a few ATP minor tournaments and was making £20,000 a year after travel/coach expenses, I imagine that I would be involved in fixing at some point in my career. The lifespan of a pro-tennis career is pretty damn short, for the guys that are slugging it out on the fringes and below they aren't making that much more than a living wage..... when you can make £15,000 for fixing a single first round match at ATP Bucharest (or more if you know betting well and orchestrate something a bit more creative) it's a huge temptation. Some of these guys have a wife/kids to think about, or just an uncertain future after retiring.
Well, the truth is both of these ways of cheating are a way of earning money. Fixing has become so common on the tour especially at these lower ranks, that it has almost become acceptable. I really have had the same reaction to it as you do: this player probably had no chance to win the tournament, so let him make some money, but maybe that's not the right way to think about it. Your example of the 140th ranked player is an interesting one because that 140th ranked player could take the fixing road to earn a profit, but he could take the doping road as well. Let's take Mariano Puerta as an example, entering 2005 ranked 132nd in the world. He could have found some people to work with, put on a good acting job and done nothing on the court to earn some money. But he decided to use performance enhancers to get ahead. Remember that this is not body building or weightlifting, and Puerta still had to go out there and work hard with his touch, with his game tactics and with his mental stability to earn that money. Some say that the levels of drugs in Puerta's blood were not even significant enough to give him an edge, but because some drugs were found, we still discredit all of his achievements. For the sake of argument, I say that Puerta taking some drugs and still having to play tennis every day is not as bad as if he took some money and became an actor out there instead of a tennis player.