It's common knowledge that the 2.4 GHz band was set aside for industrial, scientific, and medical (ISM) purposes due to its use in microwave heating. From what I've found, the United States proposed the creation of international ISM bands at an ITU convention in Atlantic City in 1947. Page 464 of this PDF of scanned documents from the convention (source) contains the U.S. proposal, which suggests five frequency bands centered at 13.66 MHz, 27.320 MHz, 40.980 MHz, and 2.45 GHz. Page 270 contains a concurrence from the French delegation, which proposes 13.32 MHz, 26.505 MHz, and 39.96 MHz. (There are current ISM bands around the middle of these proposed frequencies.) However, the 5.8 GHz band is not in the ISM proposals. The radio regulations (PDF page 57) that came out of the 1947 meeting list 5.65 - 5.85 GHz as an amateur radio band.
The ITU's search engine is pretty terrible, so I haven't been able to find when or why they declared 5.8 GHz to be an ISM band. A more recent document dated (1994-2007) suggests that 5.8 GHz is used for "environmental space heating" "bulk thawing and cooling", and "RF lighting", but I haven't found many mentions of those on Google. There's a mention of 5.8 GHz being used for wireless power transfer, but all of the examples are 2.45 GHz and below.
So why was 5.8 GHz chosen, and when did it happen? Microwave heating is apparently pretty flexible, so why that frequency in particular? I'd have expected 4.9 GHz, since it's a harmonic of 2.45 GHz. Is 5.8 GHz heating commonly used today, or is there some other common ISM usage of that band?
EDIT: To clarify, I'm not asking about the use of the 5.8 GHz band for unlicensed telecommunications, or about the purpose of ISM bands in general. I'm asking specifically about ISM (non-comm) uses of the 5.8 GHz band, and when it was designated for that purpose.