Is there any in-universe reason for a stormtrooper to use a melee weapon against Finn?

Question

It's a well-worn cliché of film and TV that, when facing off against a main character, villainous goons will use whatever strategy and weaponry makes for the most visually impressive fight scene that the hero could conceivably win, instead of simply all repeatedly shooting the hero in the chest (feel free to add a TV Tropes link in if this has a name, I'm sure it must).

But is there any explanation within the logic of the film for the scene outside Maz's Cantina, where a stormtrooper squares off against Finn (who I believe was already nervously wielding a lightsaber), calls him a traitor, and switches his blaster to electrified/melee/crowd-control mode - fighting him hand-to-hand in a duel Finn could win where this stormtrooper was the only combatant without a lethal hand-to-hand weapon - instead of simply shooting him?

I don't remember seeing any clue that the stormtroopers had planned to take Finn alive. The other stormtroopers didn't seem to be holding back from shooting at him, with characteristic accuracy.

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I seem to remember seeing something (I think it was a "riot control stormtrooper" toy?) that implied that this stormtrooper might have been carrying only riot control equipment, that couldn't be used as a blaster - but that leaves us with the questions of a) what someone equipped for riot control was doing on a battlefield, and b) why they chose to challenge the one person on that battlefield who could fight back in a hand-to-hand duel. It really looked like a blaster being switched into an alternate mode to me, which would explain question a) - but not b).

Answer 1

According to a prequel novel "Before the Awakening", the First Order stormtroopers:

• Were trained with melee weapons

• Were specifically instructed which melee weapons were effective against a lightsaber.

The instructors demonstrated the use of each weapon, the vibro-axes and shock staffs and force pikes and resonator maces, elaborating at length on the respective strengths and weaknesses of each and when and how to employ them to best effect. They explained the composite alloys used to make the weapons, how some of the equipment was strong enough to block even a lightsaber. FN-2187 wondered about that—not whether it was true but whether or not they would ever be expected to fight someone who used a lightsaber. According to the First Order, the Jedi were extinct.

Note that in the film, by the time that trooper sees Finn, Finn already has his lightsaber out and already killed one attacker with it. So, he knows he's fighting against lightsaber-armed opponent, and as his training taught him, chooses the correct weapon (Of course he had no way of knowing Finn wasn't a real Jedi and could be shot despite wielding a blaster-deflecting weapon).

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The Foster novelization doesn't really shed much more light into the attacker's thought process but confirms he was wary of the lightsaber:

No one noticed the troopers who had come up behind them—except Finn. Charging, he surprised one trooper with the glowing blade of the lightsaber, then another. A third came at him with a close-quarters weapon and the two locked in combat. Despite lack of any training with a lightsaber, Finn was athletic and courageous. In tandem with such traits, the saber made him a formidable fighter.

...

The trooper who had engaged Finn was big, strong, and agile. Finn realized the fight would have long since been over if not for the trooper’s regard for the lethal potential of the lightsaber. That didn’t stop him from finally knocking Finn to the ground and raising his own weapon for a killing strike

Answer 2

The weapon the stormtrooper uses against Finn seems to be specifically crafted to defend against a lightsaber. It shows some type of deflective energy field along it's "blade" end that resists being sliced up by Finn's saber and makes the typical "clashing energy blade" noises.

Most likely, the First Order Stormtroopers have been trained to react to the presence of a lightsaber being deployed in battle. We know that, against an actual trained Jedi, blasters are not only useless but potentially deadly to the shooters themselves. At the time, Finn's opponent has no way of knowing just how well Finn might be able to use the weapon, so he immediately switches to his anti-lightsaber weapon.

Answer 3

this is both an in and out of universe answer. I don't have the episode 3 dvd handy but I clearly remember in the commentary, George Lucas saying that Grevious' guards (in the beginning when they are rescuing Palpatine), had similar looking weapons that were designed to fight lightsabers.

Eventhough it doesn't make much sense for the trooper to fight in this way, I assumed they were just trained that "if you see a lightsaber use this thing" rather than to think critically.

My thought was that this is both a canonical aspect of the universe (Kylo Ren maybe heard legends/seen schematics about these weapons) and that J.J. Abrams wanted to "pay tribute" to the more show-y "Wu Shu"-like acrobatics of the prequels. I thought it was simultaneously bad ass and funny in that way.