Well in my Country you have also pay URA if you buy a HDD because you can saving copyrighted Material on it, but the shops don´t pay it with the argument that, old radiocassetes also haven´t URA but you can also save Copyrighted Material on it.
I don´t think that the URA will make it so much higher but if you look on various good HD Camcorder, they are Cheaper than the 5DIII and can record longer, and also have the URA included; So yes doesn´t make sense,
But maybe it´s because they don´t want to let you pay extra only because you have to click a second or third time on the record button, you only miss one or two seconds, so in my opinion it´s Ok if you don´t can record longer.
I also think How long does a clip goes, Most users press the record button and after a few minutes they say I´ve what i want, and stop the record;