I'm not sure I completely understand what you are asking.  However, Section 9 of RESPA states:

No seller of property that will be purchased with the assistance of a federally related mortgage loan shall require directly or indirectly, as a condition to selling the property, that title insurance covering the property be purchased by the buyer from any particular title company.

Applying this section gets a little bit murkey where the seller pays some, but not all, of the title insurance premium.  Regardless of who pays for the Owner's Policy, the buyer is most likely still paying for the Lender's Policy.  Thus, it would seem that if the Seller requires a particularly title company, Section 9 has been violated.