First it was a CHOICE not a mistake.
You are correct. Bristol Palin made a choice to have premarital sex... unless she was date-raped by her boyfriend, in which case she should not be marrying him. (Yes, boyfriends can rape their girlfriends. When she says no and he says "You don't really mean that. No means yes" and goes ahead anyway.)
There are plenty of reasons why a 16 or 17 year old girl may want to become pregnant and have a baby. For one thing, it conveys a certain amount of independence from her parents. She will now be on equal footing with her mother in terms of being a mother - a very important role for women. For another thing, she may have wanted to marry her boyfriend and either he or her parents did not want that. Getting pregnant would accomplish her goal. Or maybe she sees how Angelina Jolie is continually lauded for having babies and scrutinized by the press for any signs of a telltale 'baby bump.' It is cool to be pregnant nowadays. Everyone's favorite stars are appearing pregnant on the front pages of magazines, or selling the panted-after "first baby photos!!!!" for millions of dollars. Celebrity pregnancy has never been more publicized or lauded as it is today. Maybe Bristol just wanted to be cool.
Maybe she wants to be a mother. Nothing wrong with that -- it is a very important role which many females have desired since they were themselves children. Maybe she wants to be a wife and a mother and she wants to do it now and having sex is, after all, the only way to accomplish at least the motherhood part.
At any rate, absent forced penetration from her boyfriend, Bristol made her own choice. It was no mistake if she willingly had unprotected sex with her boyfriend.