EA_Bastion is correct, this is printed on the box and EA have the right to disregard your concerns and stores have a right to disregard your request for a refund.
However, that doesn't make it right. This type of mandatory install is unheard of and it shouldn't come as a surprise that Xbox 360 Slim owners didn't read the print, nor are they wrong for assuming it would install to their system without problem because to be frank, it should install. This restriction on the Slim is the biggest load of bollocks and that small print is the worst excuse I have ever seen. Poor business practise regardless.