Because they likely use the same API for their website/app/Alexa/etc. platforms, since so long as the same calls need to be made, it makes sense to reuse the work you've already done.
So long as they have reasonable rate limits & security in place, it makes sense to allow anyone to use it to order pizzas from whatever platform they want, because it's still money in their pockets -- not to mention free press from articles like "how to order pizza from vim"
