Actually, what is forbiden is to rent disc that are for not rental. The price to buy a rental disc is much more than the regular shop price, because it includes the royalties. The mention "for rental" or "includes the rent taxes" must be on the back cover of the DVD if you take it in a rent shop.

But sometimes the regular shops receive discs that were originally for rental for availability reasons, and of course they have the right to sell them.

The only (important) thing is that sometimes the content of the rental DVD and the regular one is not the same (bonus can differ, and DTS tracks are gerenally missing on rental disc). So be carefull !