Since the government is so opposed to readers, thinkers, walkers, and slow drivers, why does it allow the procession of men along the railroad tracks to exist?
I would say because they would know that there are always those in society who exist outside of it's norms. By allowing the railroad track walkers to exist they are keeping them seperate from the rest of compliant society without any effort or cost.
The government would have little to fear from the walkers as they would remain apart from the mainstream through choice, thus diminishing any chance of infecting society.
Governments have a tendency to consider things mainly in the short term, therefore their thinking or planning would not take into consideration the rail walkers eventual purpose of relaying and replication of books to be a valid exercise. It would be seen as neither attainable nor threatening.