In order to spice up the show and make people keep watching it went something like this:
Jason to Producers (or whoever is in charge of this stuff): I love Molly I am going to pick her.
Producers: How about this...you pick Melissa and propose and have America fooled until finale night.
Jason: But I don't want to be with Melissa.
Producers: You don't have to be. We will let Melissa and Molly in on the deal. Since we have to keep the winner secret anyway and you can't be seen with her, you can have your secret relationship with Molly instead of Melissa.
Jason: What's the point?
Producers: When the finale show comes around you are going to break up with Melissa and profess your love for Molly and you two can live happily ever after. Great for ratings and discussion (case in point-this blog)
Jason: I don't know...
Producers: You signed a contract and so did they...sucks to be you!
In the meantime they were all signed up for some mediocre acting classes to prepare for this dramatic "after the final rose ceremony" episode. Melissa wasn't really dumped, she wasn't REALLY chosen to begin with and Molly jumped back into his arms because they have been together all along!