Reminds me of a similar marketing gag in the Netherlands ... wasn't it The Record Store that will accept guilders as long as the DNB changes them into euros? ;-) Actually I think it is a nice service since in most or all euro countries the places where you can change old currency into EUR are open mon-fri only, and usually close around 13h ...
As for using labels in both EUR and DEM, several chains here do that (Schaffrath/furniture, for example), primarily to "prove" that the prices have been stable. Well, if they think that helps, I don't see any reason for not having such dual labels. Why the heck should it be illegal?
Christian