Lots of places seem to do this. I get emails saying get free postage if you spend over £20.00. but everythng on sale is £4.99. So you always spend nearer £25.00 to get your free postage.
However, with companies like M.&.S. you have to spend £50.00 for free postage,but returns are free. So, I suspect a lot of people order over £50.00's worth of goods to get the free postage and then send some items back. This has it's problems too, because once you have the items you just might decide to keep them all, which, I suppose is what M.&.S. want you to do.