It is. They will collect your personal information, and make it available to their parent (Telefonica, via O2) and one of the other subsidiaries set up specifically to do this, and then re-sell it to the highest bidder. See http://www.fastcompany.com/3002010/telefonica-sell-customer-... for example.
If you then subsequently leave them for another network, you will get more SMS spam than you can possibly imagine.
If you then subsequently leave them for another network, you will get more SMS spam than you can possibly imagine.
http://zingongle.wordpress.com/2012/08/22/o2-selling-my-deta... for more details.