Maybe it's an option from TTB wholesale that resellers can use or not.

Just in the process for signing up for P8 and spotted this in their T&Cs:

3.18 We filter sites that have made it onto the IWF blacklist http://www.iwf.org.uk/members/member-policies/url-list
and any websites that we�re required to by UK Law.

So there it is in black and white.