Most ISPs use the reputation of the computer that's sending a message as one of the most important factors in deciding whether a message is spam or legitimate mail. For their purposes, "reputation" means, what percentage of mail coming from that computer is spam?
For nearly all accounts, over 80% of all the mail Pobox handles is, objectively, spam. To your ISP, it does not matter that you have asked us to forward it there. They only care that 80% of the mail coming from our computers is spam.
If you could remove Spam Protection from your account, all mail from Pobox could be rejected by your ISP, and not just for your account, but everyone using that ISP. To ensure that all Pobox customers receive their legitimate email, there is a required minimum amount of spam filtering for all Pobox accounts.