What @minzinger said. We're not imposing anything - Tapastic is an open platform so you can upload any comics in any styles of any lengths. We never ever take down or bury content unless it voilates our content policy.
As for vertical format - I'm not sure if it's "Webtoons format." Many, if not most, of popular webcomics are adopting the "infinite canvas" model - Think The Oatmeal, Zen Pencils, Loading Artist, or any of popular comics on Tumblr. I think vertical is only one aspect of the larger question, ie. whether the content was created with predominantly digital consumption in mind or print consumption in mind.
One thing to keep in mind here is that Tapastic is a two-sided service and there are two types of users that we want to serve well - creators and readers. (This is similar to Uber having two types of users, drivers and riders.) So naturally we're constantly thinking about providing the best user experience for our readers. Majority of our readers now access our service from mobile. And we know it's not the most ideal experience when users are presented with comics that have never even considered mobile use case at all. That's why we definitely encourage creators to optimize their comics for mobile use case, or at least think about mobile use case. However, that, again, certainly doesn't mean that we will take down or bury comics that are not optimized for mobile.