This is in the nature of an educated guess -
I think the policce are ken to break down some of the barriers between themselves and the public, so although they wear a uniform to be instantly recognisable, the more 'formal' aspects of it are gradually being left out.
Police ties were always the 'clip-on' variety to prevent anyone strangling the officer with his own tie - so it is no real differece to miss it off.
Young people especially react differently to an obvious 'authority' figure, and this may be an attempt to 'humanise' the average officer and make them appear more friendly and approachable.