Using another analogy, it's like "vegan lasagna with meat". We all understand what it mean, but once you put some juicy meat there you can't call it vegan anymore.

I like that analogy even better. Adding some shit to a brownie ruins the brownie but doesn't take away it's "brownie-ness" in a sense. On the contrary, adding any amount of meat to a vegan lasagna instantly disqualifies it from being "vegan". There's an element of binary, black-and-white purity that "being vegan" and "being FOSS" have to have.

Non-metaphorically, I highly recommend ordering vegetarian pizza with extra meat. You get a bunch of excellent vegetables and meat to boot! Very delicious.

the only thing wrong with this analogy is that it's more of a giant spoonful than a sprinkling

Calling it "natural addative" would be more similar to how the Commons Clause is named.