I agree with daddyh2o as well - lawsuits and liability.

down here, you can do a wreck dive, have an SI on the boat for an hour, then do another dive. They allow two tanks - one for each dive. at least that way they know that all divers are starting off with 3000psi in their tanks at the start of the dive. plus the added bonus of preventing a possible accident....

just my two cents