Is there any confliction in dataset and sample period description here?
Dasgupta in the study says he is using also other event windows in dynamic estimations:

To  explore the  dynamics  of  the issuance  activities  and leverage  change,  we  create  dummy variables corresponding to the following windows around the treatment  year:  from 1 to 4 years before the  treatment; the  treatment  year  and  the 2 years  after  treatment; the  next 3 years; and  the years beyond.

with window 1 to 4 years before treatment it makes sense.
Dasgupta also uses diff-in-diff and for that you need to test for pre-treatment trends so that also can be a reason.
Dasgupta also says that cartel data are from some 2013 publication so probably there was no point in getting more data from compustat. If they miss any data for their main variable of interest after 2012 then they can't run regression on longer than 2012.