\\What has changed to make the carrying of these items both socially unacceptable and also illegal?\\
What a good question.
Society has changed, becoming a more violent society than half a century ago. Yes there was violence, the Krays, Crippen and one could go on, but the violence has spread to all aspects of society, no group are immune.
Boy Scouts did indeed carry sheath knives, a sort of badge of office to be used at camps, but I never remember any violence or crime related incidents.
The other aspect of the Scout Association was discipline and the prestigious attainment of "bettering " oneself........striving to achieve the rank and badges of the Scouting world.
I was a Boy Scout and then Sea Scout, went to all the camps and thoroughly enjoyed the comradeship and the competition.