why do these clamp on to the plug , then become stuck on and wont unattach?

Theres a spring clip that closes around the hex part of the plug. Believe me, its a GOOD thing they don't come loose.

if you want to change a plug and the dang thing is stuck on , what do you do?. i had to destroy the plug end
to remove it , it was impossible to free up

It takes some force to get them off but they do come off if you pull up squarely and apply force. 3W is the 10mm design that everyone else copied and there are thousands in use.