I don't necessarily think it was entirely a sham. I'm sure they were pushed into getting engaged, but I also believe Bristol and Levi probably tried to make a legitimate go of it. Unfortunately, it didn't work out (as with most relationships involving people that young).
It's best not to pick at them personally. The real issue here is the utter failure of abstinence-only education. A $0.75 condom could've prevented these children from having a baby before they were ready.