I believe it tends to be more about ignorance than anything else. Some ranges around here allow machineguns and some do not simply because the owner has little or no knowledge of NFA weapons or machineguns. Most of the public ranges have a rule against "rapid fire," even with a semi-auto, though it is not really directed at machineguns. It's to keep the low skill folks from hosing the range and fellow shooters with their AR trying to shoot like they do on TV.
AFAIK, at least in this area, a gun range is a gun range and there are no extra insurance requirements if they allow machineguns. There are no local or state laws that prevent them. Strictly up the the owner of the range.
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