They are not performing an invasive search of a location where you have a reasonable expectation privacy, they are performing a stakeout of a particular group of people in a busy train station; you may ask one of these people for directions and become a brief subject of interest but the observers are looking for patterns more than specific interactions.
If you don't want people to know who you are communicating with then stop putting publicly-visible txp headers on your packets :)
I'm all for defeating these creeps through mix networks, but to use that avenue as justification for mass surveillance is pretty terrible.
When somebody picks up a phone, they most certainly do have a "reasonable expectation of privacy". The right of free speech certainly includes anonymous speech, and forcing the communications carriers to install bulk snooping gear is a wholesale infringement of this right.
Perhaps a better analogy would be forcing all laundromats to install sensors that detect chemical compounds on dirty clothes.
If you don't want people to know who you are communicating with then stop putting publicly-visible txp headers on your packets :)