Why *should* he? Perhaps he hoped he could play, didn't want to withdraw, needed to see how he felt while playing. It's not as though he had a lot of time before the tournament to see how he managed the surface change. Also Lendl was here, he won't be in Cinci - another reason for seeing how things went. And do you seriously think a player of Cipolla's abilities could "do something" here? Beat a player of Raonic's capabilities when he couldn't even get a set off Murray on one leg?
This is a tournament where he usually (forget last year!) does well. However, with knee problems, Cinci next week, USO round the corner, and the weather forecast looking like having to play two matches on at least one if not two of the days if he managed to keep winning, this seems a sensible decision.
This. Murray should do what he thinks is best for himfself, why should he care about Cipolla?
If Cipolla can't beat an injured Murray that is his problem.