There are a number of reasons why you shouldn't rub your dogs nose in their "business"..
Firstly, if some time has passed after the accident (I'm talking over 5 minutes), the dog has no way to of associating your displeasure with their previous accident. This will create fear and mistrust in your dog (towards you) who will likely in the future go off an do his business in private somewhere away from you i.e. in another room of the house!
Secondly, I agree it is cruel... If a dog has an accident in the house it is the owners fault not the dog... he is just doing what comes naturally... If anyone should be rubbing their nose in it, it should be the owner for lack of attention!!
Anyway that is my 2 cents worth... some other people with more experience might be able to give you some more reasons.
