SPs who DFK don't do it with everyone..
DFK is a really personal service. Here are some reasons the ladies offer it only on a YMMV basis.
Some pooners have rotten breath or halitosis.
Some can have visible sores aroun their mouths.
Some have scraggy beards which may turn off the SP.
Soem guys are too rough or rude, so no extras for them.
Some just don't know how to kiss a lady at all.
Some guys try to jam their tongues down the ladies' throats without asking permission.
SPs don't want to catch the flu or common cold from DFKing if it's going around.
Also, at certain studios, (wo)management disapproves of their girls offering DFK.
And the girls who don't offer DFK to the general public, but only to certain clients, don't know which pussies your tongue's been Datying, so they're leery.
hope this sheds some light on the issue.