The difference is;
If you refuse to let your bag be search at a pop concert or football, you will be refuse admission.
If you were to refuse a search under the prevention of terrorism act, you can be arrested of obstructing the police.
I'm sure your answer will be, well don't refuse then. The point is, in a free country, you should if you are innocent of any crime, have your liberty curtailed in this way.
British Law and justice managed very well for hundreds of years, through two world wars, without measures that are in the POTA 2000.