I suspect it has something to do with the potential for a chip to be classified as USML Cat XII(d) under ITAR, more specifically, "specially designed for use with a null steering antenna, an electronically steerable antenna, or including a nullsteering antenna designed to reduce or avoid jamming signal"[1].

The actual USML[2] is a bit...verbose.

[1] http://insidegnss.com/u-s-eases-export-regulations-for-gps-r...

[2] https://www.ecfr.gov/cgi-bin/text-idx?node=pt22.1.121#se22.1...