If pot is legal, how will police arrest dangerous criminals?

The Boston Herald runs an article about police complaints that the decrim law is hard to enforce.
This was the kicker.

Meanwhile, in Braintree on Monday night, police spotted a suspected perv smoking pot in a car filled with coils of rope, a pair of handcuffs and bottles of NyQuil. But they had to let the man go, even though he was awaiting trial on child sexual assault charges. Said Deputy Chief Russell Jenkins, ‹Had the law not been changed, he absolutely would have been placed under arrest.Š

First of all, I’ll believe it when you show me pictures.
Second, are they really saying that they need pot laws as an excuse to hold people for other crimes they haven’t committed?
This is pathetic.