Here are a few reasons from the horses mouth
1) some feel pressured to give and gfe To most of the customers ,with the agencys that tend to provide it as a norm..
2) they can refuse a customer, and certain services easier as an Indy..
3) more money as an Indy
4) and some low level scum agencys... If the girl doesn`t sleep with the owner.. She won`t be promoted as the top list...
Won`t be offered to the customer, or literally won`t even be scheduled as working on the site..
( these are strictly the statements of a few providers, and in no way reflect my opinion.. Nor are they accusations on my behalf )
as for the risks of STD infection.. One would be stupid to think that there is more or less risk as an Indy or agency gal
and even if it`s the norm.. I find it hard to believe that the girl can`t refuse a service at will.. If a client seems infected, or unclean..?