This may be a hold over from 3.5. Some skills like slight of hand are trained only skills. Does these mean someone can't make a dex check to pick a pocket or obscure some act?
What would be the drawback of houseruling that pick pocket or slight of hand can be used untrained? Set the DC higher or have a dex requirment.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.