Don't know, but it's fairly common.  There's been a thread or two about this
behavior in the past.  My first cat was "into" soap, also; didn't lick the
soap itself, but the bathroom sink's soap dish - she kept it nice & clean.
;-)

Cathy

--
"Staccato signals of constant information..."
("The Boy in the Bubble")  Paul Simon

Fats. They lick (and sometimes eat) plastic for the same reason. Try
switching him to a diet with a higher fat (lipid) content or buy him
some fish oil (PUFAs) and let him have a little bit every day. Make
sure you don´t buy big bags if you have one or two cats. The longer a
bag (or can) stays open, the more the food "spoils" (PUFAs and
tocopherols are slowly burned by air). Make sure you close the bag
tightly after you fill his bowl and use small bowls for the food not
to sit there too long. If you use canned and don´t feed the entire can
at once, get the remaining portion and put in a glass with a lid in
the fridge.