Many illegal sites don't charge users anything at all for free movie streaming. That's because they don't make their money from their streaming service. Instead, they make their cash through ads. Many, many, many ads.
Some sites don't make any attempt whatsoever to hide their blatant ad-based revenue scheme. They pile on pop-up and pop-under ads by the dozen, or spread huge, flashing banner graphics all over the page. Each time a visitor clicks one of those ads, the site operator receives a bit of revenue.
So when you stumble upon a site that seems to be more about obnoxious, cheesy commercialism than professional streaming, you can bet that it's not legit.