If they're models, they're likely getting paid to promote a product, the brand, and themselves. Sex sells and by posting sexual pictures of themselves, they can increase their awareness and fanbase and make more money. It's kind of sad though because the model will then think that their own self-worth is based upon their appearance. Seriously, have you seen those models who made headlines and created controversy when they post pictures of themselves with no makeup and state that this is how they really look and that all those good pictures were thanks to angles and lighting?
well i guess it´s just clever click baiting... ass or titts brings you the most "likes" or "views" and since that´s probably what you get your money for at the end of the day, it´s the most logical thing to do... even if it isn´t ethical.