Why aren't adult film makers ever charged with prostitution and pimping?

I wonder because:

(1) Prostitution, also known as paying for sex, is generally illegal.

(2) Adult film stars sign contracts to be in movies where they have sex. Thus, they are payed to have sex.

How do the makers of these types of movies avoid that kind of legal trouble?

Search, and you shall receive:

http://boards.straightdope.com/sdmb/showthread.php?threadid=3935
http://boards.straightdope.com/sdmb/showthread.php?threadid=82797
http://boards.straightdope.com/sdmb/showthread.php?threadid=54608
http://boards.straightdope.com/sdmb/showthread.php?threadid=16003

They have permits to make same.

Only a couple of cities or so in the US issue such permits.