He just told you there _is_ a reason to do this: stop said ISP's users from sending spam. In the best case scenario, the ISP analyzes each email and drop them when their spamminess is too high. That's a legitimate concern for the ISP.
Now _of course_ doing this will be a massive problem for privacy, but again, user's privacy comes after a functioning network for the ISP.
I'd argue that spammy e-mails are fine as long as you don't send too many of them. That's easy enough to track without breaking encryption. But it also makes it difficult to distinguish the officially "non-spam" yet still awfully spammy marketing automation that makes the ISPs boatloads of money, so I can see their tradeoff.
I find these attacks outrageous and an absolute disdain for the protection of their customers but I wouldn't go as far as justifying spam. To be honest I am less shocked by the fact that the ISP do this than by the fact that it is even possible. Email is broken in so many levels.
Now _of course_ doing this will be a massive problem for privacy, but again, user's privacy comes after a functioning network for the ISP.