Because that is what animal right activists have manage to convince people off.and like any good myth it gets perpetuated from one person to another and if you repeat it over and over on the internet than it must be true
What the law in the UK boils down to is this this practice is currently legal so long as not practiced in a public place. Under the 1911 Act it is not illegal to feed a live vertebrate to another unless you cause it unnecessary suffering.
Now to come back on topic instead of trying to tell OP what HE should feed HIS animal, I will answer his question.
When they eat live literally it is something pretty rare with BP at least compared to colubrids, it usually occurs with young inexperienced animala or if the prey is too small making it hard or impossible to constrict. Nothing alarming they grow out of it.