Well I’m not sure how snail mail works but I think the whole point here was of encryption. With Proton Mail, the Government even if they wanted couldn’t see the contents of the mail, on the other hand they could’ve if the person was using Gmail by sending in a request to Google. In the later, even if they couldn’t find out who it was (since they were using a VPN) they could have atleast stopped the bomb blast which in-itself is a big thing.
I don’t really know who’s right or not in this case, national security surely is a matter of concern but at the same time in todays world we keep on getting farmed for data by big companies.