I'm against it, but the Philly Cops have been claiming (as a result of a couple well publicized "zero tolerance" arrests of elementary school kids for posessing sharp pencils (oops, nail clippers) that kids are cuffed for liability reasons.  Someone in some underwriting department somewhere has decided that a cuffed passenger in a police car is less likely to be awarded damages than a non-cuffed passenger.
OTOH, I think that is complete BS- the trend toward heavy handed arrest of children is part of a deliberate campaign to enhance the fearfulness and servillity of the population.