According to 47 C.F.R. 73.682(d):
(d) Digital broadcast television transmission standard. Transmission
of digital broadcast television (DTV) signals shall comply with the
standards for such transmissions set forth in Advanced Television
Systems Committee (ATSC) Doc. A/52, ATSC Standard Digital Audio
Compression (AC-3), December 20, 1995 and ATSC Doc. A/53B, Revision B,
with Amendment 1, ATSC Digital Television Standard, August 7, 2001,
except for Section 5.1.2 (``Compression format constraints'') of Annex A
(``Video Systems Characteristics'') and the phrase ``see Table 3'' in
Section 5.1.1 Table 2 and Section 5.1.2 Table 4. This incorporation by
reference was approved by the Director of the Federal Register in
accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be
inspected at the Federal Communications Commission, 445 12th St., SW,
Washington, DC 20554 or at the Office of the Federal Register, 800 N.
Capitol St., NW, Suite 700, Washington, DC.
This references the ATSC standard as I suspected. After reviewing the ATSC standard A/53 Part 4, table 6.11, it does mention that a 4:3 source aspect ratio can display a 14:9 letterbox image and a 16:9 source aspect ratio can display a 14:9 pillarbox image.
So I guess 14:9 is an 'acceptable' display ratio at least according to the FCC.