Florida has a strange bottling law that allows growlers in 32 oz and 128 oz sizes but not 64 oz. People have tried to get the law changed but so far it's failed. How did the 64 oz size come to be illegal and what is the rationale for not legalizing it?
In 2001, Florida State Senator Tom Lee championed a bill which was passed that allowed for more sizes of beer containers to be sold in Florida.
Prior to the bill passing, only 12 ounce, 16 ounce or 32 ounce packaging was allowed. This prevented most craft beers from entering the state and to allow those entry, the law enacted stated that anything under 32 ounces or over 1 gallon would now be allowed.
The unintended side affect of this law was that anything in between that range was still banned, including the 64 ounce container. 3 years ago, there was a bill written which would allow the sale of a 64 ounce container in Florida, but it was not sent to the floor for a vote because it involved other caveats with regards to craft breweries in the state. The main reason a vote was not taken was that the legislature felt that there should be more consideration taken to assist the craft brewers. Although that bill was stopped, the movement is still ongoing to have these container sizes legalized.
It isn't that half-gallon growlers are specifically illegal as such, but more of a quirk of how the different types and sizes of legal containers are defined.
The Florida laws governing beer (or more accurately "malt beverage") container sizes specify that:
Individual containers of malt beverage sold or offered for sale shall be no larger than 32 ounces.
The above law does not apply to malt beverages packaged "in bulk" which is defined as a keg or "other individual container" containing 1 gallon or more of malt beverage.
So it isn't really that 32oz and 1 gallon growlers are legal and 64oz growlers aren't, it's that a 32oz container is the largest legal sized individual container, and a gallon is the smallest legal container for "bulk" packaging exempt from the maximum container size regulation.