First of all his name was MohammadReza Shah, not Reza Shah. Reza Shah was ruling Iran after WWI !!!
It's called business. When war comes, a country will try to sell as many of its arms to the other countries to make a quick buck. Nothing to do with friendship. There are some exceptions.
And what are you talking about? The current government of Iran has barely helped Pakistanis in the wars against India. It was Reza Shah Pahlavi who was one aiding us during the wars against India and last time I checked, he was driven away by crackpot clerics.
Second, There has been no payments, hence no "selling" in reality. So, obviously it was not for business. Iran also politically supported Pakistan.
Third, then I would say that they have chosen a better policy after knowing Pakistanis are "Namak Nashnas".