While I don't think Maci's motive was to get on Teen Mom (obviously since she was on the first season), there are TONS of teenage girls who set up the trap baby scenario, my sister being one of them and a friend from high school being another. My sister got pregnant, had an abortion that she says my dad insisted on (i.e. he was kicking her out of the house if she planned on keeping the baby) and was pregnant again a year later with my nephew - it was all a ploy to have her baby daddy leave his wife and it worked for two years until she found someone else. All at the ages of 18/19. My friend's boyfriend was dumping her, she showed up at his house randomly and got pregnant that night. The boyfriend didn't get back with her, she was saddled with a baby at 17 who would later go on to become a teen mom herself - she has lost custody of that child. Girls who are horribly insecure think that a baby is going to link them to the boyfriend forever. They have fantasies about the perfect life together - if you go back and watch Maci's episode, she moved in with Ryan expecting a happy family. When Ryan clearly wasn't going to be the father she wanted, she moved in with her friggin friends in some frat house style deal because she was already semi-famous from MTV but still was trying to get with Ryan throughout and reunite their 'family'. Maci's as devious with getting pregnant as they come. She claims PCOS as a reason he doesn't have to wear a condom but with 4 pregnancies it's clear that she knows what she's doing.
I believe Mackenzie McKee,, Kail, Janelle, Chelsea and Maci all had trap babies in order to keep their 'man'. Most of those girls are smart enough to know better and all had failing relationships with the dads. A baby put Kail in a better place (outside of her mom's house), Mackenzie was able to have babies (she had to have them very young or not at all due to her diabetes), Chelsea tried to salvage what she had with Adam, Janelle thought Andrew was hot and that he would stay with her and provide her a new life outside Barb's house and Maci was clearly with a guy out of her league who had waning interest.