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If they were informed he was to commit armed robbery, they knew the clerks life was in danger. They knew he was carrying a deadly weapon and might injure or kill an innocent person in commision of the crime.

They decided that it was more important to get a conviction for armed robbery than to protect the clerk. Wrong choice.
The clerk's life was worth more.

If you or I knew hours ahead that a person was going to commit armed robbery and put a person's life in danger, did nothing but watch as they did it we might face charges of being an accessory before the fact to murder if the clerk were killed.

The police thought it was more important to get him convicted on a more serious charge, than to put him in jail for years on a lesser one. This choice could have resulted in the death of the clerk.