Wow! I really applied myself to looking for a motivation for regulation but failed. Any hints?
Edit: I take it all back. It turns out that you only have to read 21 CFR §150.160(b)(2) to see it, and it turns out that that law is just unreasonably prescriptivist.
>(2) The following combinations of fruit ingredients may be used:
>(i) Any combination of two, three, four, or five of such fruits in which the weight of each is not less than one-fifth of the weight of the combination; except that the weight of pineapple may be not less than one-tenth of the weight of the combination.
It's against the law to make a jam with more than 5 fruits because the law only explicitly allows jams with up to 5 fruits. Bizarre.
Which is partly backed up by 21 CFR § 150.160 (e)(1):
(e)
>(1) The name of each preserve or jam for which a definition and standard of identity is prescribed by this section is as follows:...
>(2) The following combinations of fruit ingredients may be used:
>(i) Any combination of such fruits in which the weight of each is not less than one-fifth of the weight of the combination, or in which the weight of each part is no less than half the weight of any other part; except that the weight of pineapple may be not less than one-tenth of the weight of the combination.
I'm not quite sure what the right legal wording is for or such that you can comply with whichever half works for you is, but that would both allow for everything the existing law allows and allow for arbitrary numbers of fruit without requiring that they be mixed in exactly equal ratios.