American encryption laws did not exist in a void. According to this website, one person who was working on the standard indicated that the key space had to be reduced to allow for export.
Take, for example, this old article discussing French law in the late 1900s: https://web.archive.org/web/20000118230559/http://www.opengr...
French cryptography exports required authorisation if the key strength was higher than 40 bits. With its 80 bit keys, the TETRA key space would've been too big to qualify for free exports.
As TETRA is part of an ETSI standard, it seems pretty likely to me that one of the European countries had a 32 bit restriction, and TETRA might as well pick the lowest common denominator when it comes to selecting a backdoor.
