The map is based on scoring in a number of categories, such as “Recognition of Marriage” and “Protected Class” (for jobs etc), and a policy that does these things give positive points and laws that specifically targets these people give negative points. 0 means that there’s not a law in the area one way or another.
One negative law could be that several states have (or have had) “Don’t Say Gay” laws prohibiting lawmakers or teachers from saying the word “gay”, for instance, Tennesse.
Yes, it is highly subjective in a number of things, because when is a law a negative law against these people or not. Sometimes it’s clear enough, such as marriage laws, other times a lack of a law might be completely intended to harm their rights, but it’s not concrete unless someone outright said that.
So basically they trawl through the laws for each state and each time there’s a law that concerns this subject they look at it to see if it is positive or negative for the LGBT people and then tally up the score resulting in this map.