A lot of webcomic artists share that view, actually. Some who have done donation drives long in the past and still, 5-10 years later, get grief from people who "pay their salary".
And in OOTS case, we've been lucky, but there have been years where we went a month between updates. He stated he can't justify a donation button when he can't promise a consistent product. Heck, even look at the Kickstarter for people who couldn't understand that it was a pledge drive and not a store front.
Simply put, a LOT of people feel entitled when they make a donation, and in the case of OOTS's lack of an update schedule, there would be a lot more upset if they donated and the comic had to end next week.