And maybe spamfiltering should be forbidden at that level. Because the entry in that field for companies is very hard. You can't buy your own spamfiltering for Google, Hotmail or ISPx. And something else is at play too: ISPs don't give much for your email. They rather overblock than underblock. It is often seen as a cost only. 50% of ISPs do a poor job at filtering.
One example: I would like a DMARC test in my spamchecker. Good luck convincing your ISP if they don't have it.
But to come back to your argument: Filtering should only be done at the request of the user/buyer of service (and/or therefore necessary for the security of the ISP).
