Nothing unusual there, though I did think the owners of the website were pushing things a little, perhaps, to ask you to install something to view content you could view for free on the official website.
Turns out I was somewhat wrong, because they're not asking you to download Zango in order to watch trailers:
They want you to agree to install Zango in order to view whole movies, some streamed on the movietvonline website from other sources, others in the form of broken up downloads hosted on file-downloading sites.
Here's a shot of what appears to be a badly made camcorder (complete with people talking and scrunching up paper in the background) streamed on the website:
Clearly, the Joker isn't asking Batman "Why so serious" - he's asking him why the camcorder rip is so seriously bad. In fact, the whole site appears to be nothing more than a mass repository of dubiously acquired movie copies:
...Holy Pirated Content, Batman!