[ZoFreX]

> Why do they need to do this?

This is explained in detail here:

http://content.tfl.gov.uk/review-tfl-wifi-pilot.pdf

If you would rather buy a paper ticket with cash you can still do that, and you won't be tracked if you put your phone into wireless mode. Personally I'm happy for them to collect limited amounts of data, with restraint, if that means they can improve the effectiveness of the network. But if you're not, that's understandable, and it's not that difficult to avoid it.