so what you are saying is that if I buy a second hand iphone it might be useless to me because of this weird 90-day limitation as I cannot register it to my account even if I apply full restore/reset. Then I should not buy it.
I always thought this limitation is to prevent exploiting purchase rights, but what you are saying is far beyond reasonable. Apple is one of the least customer oriented companies, this shouldn't be a surpsise though.
"so what you are saying is that if I buy a second hand iphone it might be useless to me because of this weird 90-day limitation"
No. You can associate another iphone to the same account. You just cannot associate ONE DEVICE to another account until 90 days after you associated it with the first account.
It has NOTHING to do with the number of devices associated with one account.
"I always thought this limitation is to prevent exploiting purchase rights,"
You were right.
You seem to misunderstand completely now.
You can assoiciate up to ten devices with one itunes account. Once you associate a device to an itunes account, you cannot associate it with another itunes account for 90 days.
I think you actually completely misunderstood what this guy is asking.
He could not actually associate this device with his account because it is a second-hand phone, not a second iPhone for the same account. He was correct in what he said because it's already associated to his friend's account.
Maybe try not to be so rude in help forums.