Everyone has a reason (not usually a good one) to justify why they deserve special treatment
Just like I didn't see a reason to exclude the sales of nonprofit performances (such as at a nonprofit theater) from admissions sales taxes--after all, these groups get monies from the city and these monies need to be replenished--I see no reason for food trucks to be exempt from DC food sales taxes. See "Food trucks may lose their sales tax exemption" and "D.C. food truck owners object to sales-tax proposal" from the Washington Post.
Flickr image by BrianR. You can love food trucks and still believe that their sales should be subject to sales tax.