In November 1927, Charles Moore, Chairman of the Commission of Fine Arts sent a letter to Mary O'Reilly, Director of the US Mint, requesting that the mottoes "IN GOD WE TRUST" and "E PLURIBUS UNUM" be left off the Hawaiian Sesquicentennial half dollar. The request was based on the Commission's belief that, while fine on regular coinage, the mottoes "disturb the special design" of a commemorative coin.
Moore stated his belief that the bill/Act that authorized the Hawaiian coin would need to include language that specifically allowed the omission. Moore was partially correct in his thinking.
The motto "In God We Trust" first appeared on the US two-cent coin of 1864. As no law was put in place to require it, the motto's use on
US coins in the years that followed was inconsistent, appearing on some but not all. The initial issues of the US commemorative coin series did not feature the motto; it makes its first appearance on the 1915 Half Dollar and Gold Quintuple Eagle coins struck for the Panama-Pacific International Exposition (the program's Gold Dollar and Gold Quarter Eagle do not include the motto). It began to be used regularly, though intermittently at first, on US commemorative coins from that point forward. (See
Quick Bits #92 - The "Godless" Issues for an illustrated survey of US commemorative coins without "In God We Trust" included.)
The inclusion of "In God We Trust" on
US coins and paper money was not mandated by law until July 1955. At that time, the US Congress authorized, and President Dwight David Eisenhower approved, the bill calling for inclusion of the motto on all new
US coins and paper money. It was already a long-standing tradition on
US coinage by that time, however, and was featured already on all then-current US circulating coins.
So, in 1928, omitting "IN GOD WE TRUST" from the Hawaiian Sesquicentennial half dollar would not have required a specific provision in the coin's bill/Act - just an agreement with the Treasury Department / US Mint.
In contrast, the Coinage Act of 1873 mandated "E PLURIBUS UNUM" be included on all
US coinage. This should have impacted every US commemorative coin issued, but multiple early coins did not include it. Of the commemorative issues that preceded the 1928 Hawaiian, only the 1915 Panama-Pacific International Exposition Gold Quarter Eagle, the 1915 Panama-Pacific International Exposition Gold Quintuple Eagles, the 1918 Illinois Statehood Centennial Half Dollar, the 1920 Maine Statehood Centennial Half Dollar, the 1922 Silver Half Dollar and Gold Dollar issues for Ulysses S Grant, the 1925 Stone Mountain Memorial Half Dollar, the 1925 California Statehood Diamond Jubilee Half Dollar, the 1926 Silver Half Dollar and Gold Quarter Eagle for the American Independence Sesquicentennial and the 1927 Battle of Bennington/Vermont Independence Sesquicentennial Half Dollar included "E PLURIBUS UNUM" - 11 of 28 types/programs, ~39% of all programs.
Note: The Coinage Act of 1873 included language that stated the "In God we trust" motto may be inscribed "upon such coins as shall admit of such motto." So, an explicitly-stated option but not a legal mandate, beginning in 1873.The Public Law that authorized the Hawaiian Sesquicentennial Half Dollar did not include any language regarding motto omissions, and the coin was struck with both in place: "IN GOD WE TRUST" on the obverse and "E PLURIBUS UNUM" on the reverse.
1928 Hawaiian (European) Discovery Half Dollar

For other of my posts about commemorative coins and medals, including more about the Hawaiian half dollar, see:
Commems Collection.