Use strict will break code, if you simply enable it afterwards.
So I think it is the right decision not to do this (Enable it by default).
I think let people decide what they do even if it is probably "Not the right way or Best Practice". Thats freedom ...
I think it is useful to use strict, but it should be used from the first line of code not enabled afterwards otherwise it will break the program almost for sure (At least it does with my simple pieces of code this wont be better with large programs?) ...