Bandit: Totally justified from a typically 'high-school' ethics, in fact a very petty form of shoplifting. I know some folks who bandit and they never disrupt anybody and don't take from aid stations, etc... I think it's pretty cheap (in the best meaning of cheap - i.e. not that bad) and it is taking advantage of somebody's labor. Bandits don't typically misrepresent themselves, aside from 'pretending' to be paying customers. Bandits (insufficiently) justify their behavior to themselves by saying that the race would happen anyway, and they're just saving some money, and don't need a t-shirt in any case.
Cheater: Far worse than the above. These folks are paying but they are doing so for the purpose of misrepresenting themselves to their competitors and community (family, coworkers, etc..). Cheaters typically don't even attempt to justify their actions; it is considered wrong for the following reason. Competing is all about having fun while attempting to be a little bit glorious without hurting anybody (see e.g. sports in Homer) - cheating subverts the whole purpose of this since glory is acquired outside the confines of the (predetermined) contest. In the case of cheating, this (like banditry) also takes advantage of the race director's labor by subverting the nature of the contest.
For cheaters, it's willful misrepresentation for the purpose of elevating their own status. For bandits, it's willful misrepresentation but not for their own glorification (at the expense of other competitors). So cheating is much worse.
all in my not so humble opinion...