There is some other places with similar laws, and the point of these laws was to define "Saturday Night Specials".
The term comes from the earlier term "Niggertown Saturday Night Special".
Which was used to refer to the type of guns even poor blacks could afford. Since it is primarily low income areas that have violent crime, it was primarily guns affordable to low income people that became associated with violent crime.
In some parts of the nation the low income areas were predominantly black.
The focus of some groups then became how to outlaw the poor man's gun without outlawing the others that would cause the middle class to defeat such legislation.
"Saturday Night Special" became a prime term, and then "Junk Handgun" to avoid the racist origin of that term.
It had nothing to do with safety of a particular firearm.
The guns that typically worked well with cheaper casting methods that made them affordable to poor people used low pressure rounds to be safe. The result was they were often chambered in .25 auto, .38 special, .380 and .22lr. So these calibers, and especially the .25 auto and .380ACP used for very little recreation became associated with crime.
The origin is clearly class based and racist.
Since legislators cannot get away with saying "the guns ******* and white trash are using" they had to define it more specifically. The result was looking for some definable difference the cheap guns and the more expensive ones had. They found that in a lower melting point.
Their class based agenda then could take on the appearance of "safety".
As many polymers designs with much lower melting points have shown, the melting point of the frame is less important than insuring quality components are used at a few key areas. While the rest can be "junk".
Since Glocks became extremely popular with police in the 80s and 90s, legislators could not extend the same limitations to polymer frames without clearly outlawing a reliable, popular firearm of clearly acceptable quality .
So polymers frames are not included in the limitations.
Ironically a Glock is cheaper to make than most of the old cheap cast firearms associated with "junk handguns". They just always have charged as much as competitive handguns to regular people (though they give deep discounts to LEO), so never became associated with "Saturday Night Specials".
Glocks and most polymer frames will melt around 400 degrees. Half the temperature.
People melt the grip to add designs or stippling with rather low temperatures. Just boiling water can soften the polymer enough to reshape it (or ruin the gun or make it unsafe)!
Here is a SIG put into the oven by a police officer to dry it out and forgotten when he got a phone call:
It started out looking like this: