Makes sense but this is interesting, didn't know this couldn't be lowered. Did the 0x team provided an explanation for that design?
So play this out in your head. You stake with a pool that says they will pay back 90% of fees to stakers, great, right? Right before the payout epoch ends, they lower that to 0% and you get nothing.
At best, that is mean and unfair. At worse, it is blatently malicious. In the current construction, if a pool is offering a 90% payout, they are only able to increase that value (remember: they do this by lowering the proportion kept by the pool operator) which only stands to benefit you and other stakers.
With this construction, stakers are always gauranteed to get at least the percentage advertized, and maybe more. It's quite brilliant.