Is broner not a welterweight?
No
He is considering he didn't make weight for his last fight.
He weighed in at 141. If you wanna go that route, technically he's a Welterweight, but then you'd have to agree he's a VERY small Welterweight.
Let's look at all his "welterweight" fights, since we're gonna use it in the technical way:
Weighed in at 146 against Paulie
Weighed in at 144 against Maidana
Weighed in at 141 against Molina
Broner also started his career as a a Lightweight while Porter started his career as a Super Middleweight.
And he won that belt at damn near 147.
He won it from Paulie, who himself was a career Junior Welterweight and had moved up 2 years earlier, and had no power to speak of.
That didnt make Broner a true Welterweight.
Chinny julio Diaz 2 divisions removed from his prime weight was able to trouble porter with no catchweight, why can't broner?
Why should he have to? We are in the era of "The A-Side runs things...B-side be quiet".
Im pretty sure if Julio Diaz could still make 140, and was the A-Side, he could've demanded a catchweight. Instead he had already fought 5 times at Welterweight, before even facing Porter, including his 4 previous fights in a row. Not the same at all.
Besides, that question you just posed can be spent around and used against Floyd, Manny, Cotto, or anyone who has ever participated in a catchweight bout.