it just means it's arbitrary.
Do you like arbitrary laws?
To me it indicates that the people crafting the law have no idea what they are regulating or why - if they could produce some sort of data/study/etc showing that 7 is the "magic number" that tends (correlation) to decrease (if not directly impacts, causation) the deadliness of a mass shooting, then I'd bite.
Laws need not be perfect, but should at least be rational.
Arbitrary and rational are not mutually exclusive. For example, age of consent is arbitrary but it is also not irrational.
Perhaps you were thinking of random. A random law wold be awful.
I would like to have laws that are not arbitrary at all but often that is not possible.