It's allowed because moderators and creators of the websites like this cannot freely control whatever insane plot their users come up with to modify their numbers.
It used to be, back in the wee olden days of yore, that you would sit on your work and refresh the page every 10 minutes or so to get a new "view", and that'd bump your numbers. You also could like / upvote something without an account. However, those practices don't exist due to the coding modifications, so people resort to the other ways to modify their numbers, with straight up begging for interaction: like every thing, comment on everything, sub to everything, in extreme cases, make dummy accounts to up your own likes.
I suspect the current influx of the sub-for-sub brainwaved people are refugees from some other site where that (cough) worked. Or barely worked. Or worked in their assumption that it did. Wattpad for one I'm pretty sure uses those kind of archaic number skewing methods, but Tapas seems to have a smarter code where the amount of subs you have does not adjust to better "overall" number math in popularity or anything of the sort. In fact, if anything, a non-interactive "sub-for-sub" like these people seem to want, looks like it'll just punish the users by giving them dead subs. Dead subs means no comments, no likes, and no interaction, which will in turn translate to lower numbers.
An example of this would be my numbers when I get a binge reader. A binge reader, no matter if they sub or not, will effect my daily numbers up by both a value (say they read 50 updates) and a percentage based off of the prior day's interactions (likes, subs, and comments).
For example here, for most days, that 1 view would be a 100% + because I normally don't have two days in a row with a single view. Tomorrow, that % will read 100% - in red.
A work that has 500 subs on it in a day, and 1000 views in a day, will just be more harshly punished down the lines if those numbers do not keep up. Day one has 500 subs, while days 2 3 4 5 6 and 7 have less than 100 each, or worse, 0. Probably by the way the formula works, unless these people of the initial hoard continue to interact with comments and likes, the work as a whole will suffer.
My suspicion is that this is how Tapas has a formula that determines what is popular or trending.
So, no. It's not against the rules, but by current formulas and math done on many websites (youtube, instagram, tapas, twitter, twitch), the more dead subs you have, the more of an uphill battle you're fighting.