Just because UL says "cable for aboveground installation" doesn't mean it is not approved for use underground in conduit. It means UL tested it for aboveground use only, and they did not test it for underground in conduit use. It wasn't tested for use in a conduit in the ground because the manufacturer did not pay them to do that test.
Would you agree that it is OK to install SEU or SER on the outside of a building, sleeved in conduit? (Such as we see when we have a mast)
What possible extra harm could the SEU or SER come to when installed in conduit underground as opposed to aboveground?
The same argument could be offered for installing NM in conduit, which the 2005 NEC finally addressed. (it is allowed, but not underground because of the wet location)
I agree that a future code proposal is in order to clear this up.
