There are plenty of pokes that can utilise moves that they're weak to though. As I said, it's purely competitive reasoning but you could totally make up a flavour reason such as - it can use Flare Blitz because the firey recoil damages it, therefore fitting with the fact that Durant is weak to fire.
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jagged_angel
JayHankEdLyon
Except that Durant's entire flavor revolves around its rivalry and weakness to Fire. This is not a pokemon that can cloak itself in flames. This isn't like Dragonite using Ice Beam or Ho-Oh using Ancient Power; Flare Blitz Durant flies directly in the face of its flavor.
JayHankEdLyon
And moreover, the point of this meta is to make megas, not reshape their non-mega forms, which is precively what Flare Blitz Durant would do.
jagged_angel
The point of the meta is to make OU viable metas, and Flare Blitz is the only viable physical Fire move (discounting crap like Flame Charge and legendary moves), meaning that if it's mega is going to be based off a bombardier beetle, it needs that move.