I'm going to propose an alternate interpretation of this, hopefully the community won't downvote me for having a different opinion...

The FDA is actually acting in a manner consistent with it's stance on this issue. For example, take butter and "I Can't Believe It's Not Butter!" Ever wonder why someone decided to market a product by defining what it isn't? Because they are legally prevented from calling it butter, per United States Code, 2006 Edition, Supplement 4, Title 21, CHAPTER 9 -FEDERAL FOOD, DRUG, AND COSMETIC ACT SUBCHAPTER II - DEFINITIONS In the case of butter specifically, see Sec. 321a - Butter” defined or scroll to the bottom of this page

So, now that we know mayonnaise is very clearly not the only food the FDA has decided to define, let's address the second part of OP's assertion, that they are a corporate shill for factory farms. Well, guess what, they are.

Lol.

But really, it's sad how money/lobbying has purchased and gutted the same agency that used to protect us.. But I don't think that is necessarily to blame to for the warning issued to JustMayo (which I love as much as any other vegan). These definitions were introduced in 1906, as many here are probably aware, partly in response to publicity from Upton Sinclaire's The Jungle which highlighted how disgusting and unsanitary factory slaughterhouses and other similar industries were. Among other things, companies were selling undesireable parts of animals as more valuable parts in order to make larger margins. The law, designed to address this and other abuses of common sense, was initially called the Pure Food and Drug Act and later amended in the 1930's to the current Federal Food, Drug, and Cosmetic Act