I’m sure this rule is also to avoid being scammed by certain players. Meaning, lets say you have a player that does not like to play the bonuses due to wagering requirements. Then the player loses his/her money and tries to claim the bonus saying he did infact send the email to claim it when infact he didn’t. I could see that as good reason along with others mentioned here to have this rule.
I would be curious how often players complain about this and how often a player is denied a bonus because he started playing the deposit before the bonus was credited even though this player had no malicious intent. If this is a reoccurring problem with their players, I think that they would agree something needs to change somewhere. If this problem does occur often, then there is no telling how many customers opt not to play again because they have a bad taste.
Most affiliates are reasonable and can see it from the casinos perspective, but it doesn’t take much for a player to say ” the hell with this place, I am going to find a casino that want’s my business”. So casinos should do everything they can to take the players by the hand to avoid such situations.