Because the master password is also used on a desktop. when I'm using desktop I don't mind typing in the longer, more protective password because I have a keyboard. While on my mobile device, I do wish to set a pin which is equivalent to my master password for the sake of convenience.
So from what you are trying to say, yes, it will be easier to compromise enpass on my phone with a shorter pin, however, that's a personal choice to make, and you will have to steal my phone, crack my phone lockscreen before I remote wipe my phone, and then guess my pin for enpass before accessing my password library.
I don't believe this is a new practice that you have never heard before, a few banking apps that I'm aware of in Australia and New Zealand (which is where I am) are also using this practice on their app.