Just a misunderstanding. They only drink a little blood, and only for ritualistic purposes.
https://www.huffpost.com/entry/megan-fox-machine-gun-kelly-blood-drinking_n_626854e7e4b0ea625c0d9427
https://www.huffpost.com/entry/megan-fox-machine-gun-kelly-blood-drinking_n_626854e7e4b0ea625c0d9427