Why ParcelForce charge penalty for misleading dangerous good label

Dangerous goods need to be specially processed by licensed people before it can be taken on airplane. 
With misleading dangerous label(mark) on the box means you are declaring that your box contains dangrous goods regardness whatever insidde. 
As a not very good example, this is similar to someone claims having a bomb while trying to on board a plane. This will immediately trigger the safety alarm, and causes panic for the whole system. 
 
That is reason why they introduced the penalty for this situation.
 
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faq group: 
ParcelForce英国邮政快递