On the Travel Safety/Checkpoints-Border Policy debate forum an opinion is that Mexico is doing this now in exchange/ retaliation/ response to the political debate on immigration. However, the much more likely reason is that is being done after much long time urging by the US Govt which would really like much better information on who and how many people are departing the US.
Much ado about nothing for most people (except those that prefer not to be tracked) - no one seems to mind about document requirements to go into Canada....