Because Jason would have got out on his own eventually (seriously, apparently Arkham can't keep its inmates in for more than a couple of days in a row, how long would it have taken a Bat to get out of a normal prison?), so Tim figured "if I help him out as a gesture of good will, maybe he'll go straight".
It even seemed to be working, Jason was considerably nicer to Tim in the Cave than he had ever been. Too bad Tony Daniels got to him and ruined everything.