Because they don't want to risk the .000000000000000001% chance that somebody somewhere will now think of Doritos as "The brand of chips butt rapers like". It's their logo, they own it, they have a right to decide in what context it is used by an outside party.
Which is exactly what we find ridiculous. If your product becomes successful enough to be an everyday item, you should accept the fact that your logo is more than your intellectual property.
Hell, even calling it an intellectual property once it's been sold is ridiculous.
The reality is, they just want to make money, either through suing, or forcing you to pay them without blurring it out.
But, I will now refer to Dorito's this way... lol... if anyone asks me if I'm hungry for anything, I will say "The brand of chips butt rapers like"