Not true, but there are provisos.
If legally challenged, you must be able to prove that the snake absolutely refuses to eat pre-killed and will die unless it is fed with live mice.
Theoretically, you could still be prosecuted for cruelty to the mouse. There have been no legal test cases so far and the waters could be further muddled by the revised legislation under the forthcoming Animal Welfare Bill.
My interpretation only: don't feed live unless you absolutely have no other option - and can prove that you have exhausted all other options.