but it is possible to copy the code, modify it, then create fishing site and distribute some bad app, right? it would be eliminated, yes, but some people can suffer.
You could also say the same with close sourced apps(the distribution of "bad" apps). In fact, unethical and immoral people do that all the time. They create scammy app versions of some famous websites that doesn't really have official apps in the hopes of victims thinking that it's the official app. Like what o_e_l_e_o said, simply don't download from unofficial sources; and this applies for both open source and closed source software.