Currently we install just zone.tab, but it says:
# This file is intended as a backward-compatibility aid for older programs. # New programs should use zone1970.tab. This file is like zone1970.tab (see # zone1970.tab's comments), but with the following additional restrictions: # # 1. This file contains only ASCII characters. # 2. The first data column contains exactly one country code. # # Because of (2), each row stands for an area that is the intersection # of a region identified by a country code and of a timezone where civil # clocks have agreed since 1970; this is a narrower definition than # that of zone1970.tab.
Considering messages like  I suppose we should install zone1970.tab too for other software that wants to use it, and then we might as well use it for tzsetup(8) too.