I wouldn’t be surprised if that was technically legal according to their rules

I highly doubt those rules can override actual laws, even if they're in fine print you technically signed. I just don't see a bullet point "we can secretly make a key to your property and remove something from it without any notice" holding up.

This really has nothing to do with calories as a unit. This is all about "we'll make a serving size some arbitrary bullshit small number so we can round down and write it has zero calories". That's why here it's required by law to write calorie information per 100g. You can still write per serving size in addition to that which fools some people anyway I guess.