This must be what pro-lifers feel like when being confronted with pro-choice arguments for bodily autonomy. "But she's making the WRONG CHOICE!"
What if the wants of the surrogate and the biological parents were reversed - the surrogate wanted to have an abortion, but the biological parents were against it and the contract had decreed that the biological parents have final say? I know I would be defending the surrogate's right to decide on the basis of "my body, my choice", and would insist that any contract to surrender such a fundamental right is invalid.
What if the fetus had had a mild disability or anomaly - say, Klinefelter's? I'd be thinking the parents were asshats for wanting to abort, and again, the surrogate's choice should trump any contract. Based on the story, many of the defects were not apparent in the ultrasound.
With these in mind, I really think such contract should be invalid, and the surrogate's choice respected, even when I think she's making the wrong one. The surrogate, while within her rights, should have thought things through beforehand; but that's unfortunate thoughtlessness and naïvete, not rageworthy malice.