This is true but that doesn't mean they want to not be allowed to if they want to. This isn't an exploit so there is no reason for the rule to apply to MFN but makes sense to have as an option for created leagues. I know I wouldn't touch a league like that with a 20 foot pole and would likely leave MFN if such a cap were put in place even though I don't think I've ever traded more than 15 times in a season myself, though I could be mistaken.
This is exactly why it should be league base. Myself, along with other owners(yes I have communicated with them) have seen certain leagues that we wouldn't get in due to certain individuals and their trades practices.