If you look at the linked list, most of the big internet providers in the UK are members so maybe it would reflect badly on VM if they decided not to be members? I'm not 100% sure and you would probably have to ask Virgin themselves (not sure which department could comment on it).
More to the point, I'm not sure the IWF thing holds up, as the message displayed is the same for The Pirate Bay.
It does hold up If you here it explains why websites are blocked.
Whilst rare, from time to time Virgin Media is required to block access to certain websites meaning they cannot be accessed by customers using our broadband service. Blocking access to certain websites is something that Virgin Media does only when absolutely necessary. The two scenarios where sites may be blocked on our service are:
(a) because the IWF (Internet Watch Foundation) has asked us to block a particular site to protect children and our customers from potentially illegal content.
(b) we receive an order from the Courts requiring us to prevent access to a certain site, for example in order to help protect Copyright – this is a legal obligation we must comply with.
"The Pirate Bay's artist promotion platform (the Promo Bay), despite being perfectly legal, is being blocked by several UK Internet service providers including BT, and Virgin Media. The Promo Bay was launched this week as a promotion platform for content creators like filmmakers and musicians enabling them to showcase their talent and work to thousands of people across the web. Even though the idea is novel, The Promo Bay has somehow found itself on a block list alongside the Pirate Bay."
Sadly not a Virgin customer anymore.... Unless you want to cable my new street. my internet sucks.