If they buy game now, they have to pay for the first 3 DLC characters
Also they might have missed the DLC trial period if they didnt play the game for a long time.
There is no trial period, the DLC characters were offered for free for the first three months after they're released. Lab Zero went with this solution because it would have been too much of a hassle making sure everyone who donated got a key for each DLC character otherwise, if I'm not mistaken.
It sucks if you missed on the free period but those who didn't donate aren't entitled to get them for free. Lab Zero sends out an e-mail reminding everyone who donated that the character got released to make sure they don't miss out on them.