Yeah, this isn't cheating at all. You're getting too hung up on the "real baseball." Player gain/lose eligibility based on how they're actually used in real life. CBS does this, Yahoo does this, and I assume ESPN does too though I've never used that service.
It's all about flexibility, and about balance. Michael Cuddyer has started a few games at 2B, so he gets 2B eligibility, letting you, the manager, choose to deploy him there, or not.
With pitchers, you've only got two options: SP, RP (or P, in Yahoo, for some prospects). Now, eligibility is determined by how pitchers were used the previous year, and this year. For example, Jeremy Hellickson was used in relief last year, and he started a few games, so he's got dual eligibility. You can deploy him either as a starter or a reliever. Now, since this year he's starting 100%, you can stick him in a RP spot and get starts from him. In most leagues that's not too valuable. In fact, it's the opposite kind of pitcher who can pick up value. Default Yahoo leagues go 2SP/2RP/3P, so if you've got a SP-eligible reliever, you could deploy 6 closers on days when you only have 1 start, or 7 on days when you have none.
Or take a league with 5 SP spots and 5 RP spots, no generic P spots. You're probably not going to be able to get 5 starts a day out of your staff, so wouldn't it be valuable to have some SP-eligible relievers to maximize your stats? That's where SP/RP eligible guys like David Hernandez (last night notwithstanding) come in. Just because he wasn't used as a starter the very last time he pitched doesn't mean he's not eligible for that role. Then you have, say, Houston Street. The last time he started was probably in high school. Only RP eligible.
Anyway, point of the point - you're wrong, this isn't cheating. It's not even a "dirty" strategy. I'm kind of confused how you see eligibility SHOULD be determined - take Martin Prado. You think he should be OF eligible, right? Oh wait, he started a game at 3B. Now he's 3B-only for a game. Whoops, back to OF. Oh, moved over as a late-inning sub for Uggla. 2B now. That makes no sense.
You need to change your perspective here - being mad at CBS for doing it this way isn't the right attitude.