Not every "A is B" satisfies the definition of "the worst argument in the world" (truly a horrible name for a fallacy which should be replaced by something shorter, more descriptive and less exaggerating). "A is B, therefore A is C" qualifies as the discussed fallacy if

1. A belongs to the category B as far as technical/denotational meaning of B is considered,
2. using the technical meaning, not all B are C,
3. most (or most typical / available) members of B are C and therefore B has C in connotations, and
4. all C-relevant information about A is known, screening off potential C-relevant information about A coming from its membership in B.

In "burning fossil fuels (A) is environmentally irresponsible (B) [and therefore is bad] (C)"

1. holds
2. is subjective but for many audiences fails (i.e. "irresponsible" means "likely to cause bad outcomes", which makes the whole category tautologically bad)
3. is problematic, since the denotational and connotational meaning of B aren't different (badness-wise)
4. fails, since presumably the listener doesn't know about A's environmental irresponsibility

The argument may be fallacious if the listener doesn't care about the environment but is tricked into accepting the badness of A based on connotations of "irresponsible", but that isn't exactly the fallacy described in the OP.

In "guns (A) are weapons (B) [and therefore should be banned] (C)"

1. holds
2. holds if the listener agrees that all weapons should be banned, else fails
3. depends on the listener's idea of a typical weapon, if it is a hydrogen bomb, then (3) holds, if it is a knife, (3) fails, if it is a gun, we are building a circular argument
4. probably holds

So this argument may qualify, but it is so obviously tautological that I have problems imagining someone actually using it.