Why does the T need to retain your travel information for two years?

Gary McGath notes the new CharlieCards will have RFID chips and that the T plans to retain information on where and when you get on the T for up to two years:

... It opens the possibility of privacy invasion, both by government agencies and by private parties. What information can be read by someone with a completely placed scanner? Does it have openly readable personal information? Anyone who issues an RFID-based personal ID should tell its users that, but the MBTA is telling us nothing. A Google search for "RFID" on mbta.com turns up no hits.

I'm not a fan of tinfoil hats, but a tinfoil wallet is starting to sound like a good idea to me.

Xjustquietx, though, doesn't buy it:

... For god's sake. So they're tracking where you get on the T in the morning and where you get on at night. So now the government knows where you WORK and LIVE. Like anyone with 5 seconds and an internet connection can't FIND THAT OUT ALREADY! GET THE FUCK OVER YOURSELVES! Oh wait, this says you went to Newbury Street on Saturday. Hmmm, maybe you LIKE SHOPPING. Is that EVEN POSSIBLE? IN AMERICA? IN BOSTON!? ...