I ran into the subscription prompt whenever I tried skipping too far ahead or back in any of the comics there that I tried taking a peek at.
The fishy part that immediately strikes me is that a) they want your stuff for free b) they then turn it around and make it not for free.
Possibly they have some sort of profit-sharing scheme, but if it's anything like the one I was approached by...hm which by their own account was a Korean-styled one, apparently meaning that authors upload their comics in episode chunks, and they're read by simply scrolling down through all the episode's pages on a long single page--it was going to be comicpanda.com but has apparently now actually morphed and launched as http://tapastic.com/
...where was I...oh yes well anyway their proposal had some profit-sharing scheme, but it was almost completely lacking in transparency, relied on their very sketchy promise of getting tens of thousands of readers to your comic, and even then paid a mere pittance of what you could make on your own site with your own ads at even a fraction of that readership level. Granted, they do offer a link to your home page at the top of their listing for your comic, but their site is designed as a community thing and the purpose of community things is really to keep the people on the main site.
Now, the one you're dealing with is acting as a translation service, which on the surface is slightly different, but in most ways it's pretty much the same thing. The two main things that ultimately turned me off to the one that approached me was that a) I'd have to reformat my stuff to fit their site, which would be a certain constant level of work for me, with no guarantee of payoff and b) once I uploaded it to them, it would be out of my control, as they didn't promise to offer any kind of functions for the author to edit or delete their work once it had been sent. So it's basically doing work for and giving your existing work to a little-known publisher with no guarantee of any return.
In the case I had with this other site, I offered to give them a tiny section of my archive that they could put up as something to get their users interested in coming back to my site, but unsurprisingly they did not get back to me; what they really wanted I guess was a free, constant stream of content from me that they could turn around and sell to their subscribers. If they had actually offered something professional and up front, like a per-page pay rate, I might have considered it, but there was no mention of anything like that.
And in your case with this translation scheme, since the "exposure" they're trying to get to your work on their site is to non-English-speaking readers, there'd be pretty much zero incentive for those readers to click through to your own site.