An important part of WAitW is that it uses an atypical member of a category to attach to it the rejection of a typical member of the category. Both abortion and death penalty are atypical members of the "murder" category (if they are), and associating them with "murder" is trying to associate them with the connotation of the "typical" murder.

Guns are quite the typical weapon. They are not border-line weapons like a kitchen knife or a hammer, they are not military grade weapons. Saying "guns are weapons!" doesn't try to associate guns with something different, it doesn't add much to the debate, but it doesn't carry the same attempt to sneak in connotations as "abortion is murder!" is.

For "Burning fossil fuels is environmentally irresponsible!" it's also quite different. "Environmentally irresponsible" is more a description, a feature that "burning fossil fuels" has (or doesn't have, but that's another issue), than a broad category in which we are trying to tie "burning fossil fuels".