RESPA doesn't differentiate between an Owner's Policy and a Lender's Policy.  It merely says that seller cannot require the Buyer to purchase title insurance covering the property from any particular title insurance company. 

I suppose the Seller could argue that the Buyer was free to choose any company he wanted for his Lender's Policy, but as a practical matter the Buyer is never going to use a different company for the Lender's Policy.  If the Seller can require a particular company for the Owner's Policy, it may be an indirect requirement that the Buyer use the same company for the Lender's Policy. 

Besides, in your hypothetical it does appear that the Seller was requiring a particular title company and the Buyer is still paying half the premium.  Isn't that still a requirement that "title insurance covering the property be purchased by the buyer from any particular title company?"