The easyJet ops manual states the following on this subject:
"easyJet does not operate an indemnity policy and therefore under no
circumstances, under the Child Protection Act, must another passenger be asked
to accept responsibility during flight of an unaccompanied minor. In addition, it
is not easyJet's policy to allow another passenger to take responsibility for an
infant to be seated on their lap for take-off or landing."
The captain involved complied with a black and white company rule. This rule has been created by senior management in consultation with lawyers who can reasonably be expected to know an awful lot more about the legal aspects of the situation than a line captain. There was absolutely no excuse for the captain to override this particular rule, and he would have been completely out of his mind to do so.
Feel free to condemn the management, the legal system or even the government if you disagree with the rule. Do not condemn someone for doing their job right.