Not just Johnson - that's also the interpretation in legislation. 'Should/may' in legislation has a different meaning to 'shall/must'. Although to be fair 'should' generally means 'ought to' rather than 'can do'I am reminded of the Johnson interpretation of difference between 'should' (its up to you, do what you feel is best) and must (you have to do it).
Perhaps in football its the other way round. And 'get in the sea' is an invitation to join Judith Chalmers on a summer holiday in the Maldives.
There was one (England player?) who was booked for not picking up the nearest ball to take a throw-in, but going two steps further to pick up a drier one.I have never seen anyone booked for not taking a throw in.