I can understand it in one sense, if they continue to ride and are then found guilty it could call into question anything they have done since. But i'm with you, seems a lack of trust by their current employers !
It seems like the teams are in a bit of a 'damned if you do, damned if you don't' situation. Drop the riders and nothing emerges and you've deprived the riders of the ability to compete in events/potential loss of earnings etc, don't drop them and if doping is proved others are affected by their tainted results etc.
Personally I think it's to soon to drop them i.e. before there's even been any accusation. It's interesting to note that Lampre (who appear to be closer to the investigation) have not dropped any riders at this stage.