Wow you guys...just wow. Some real... talking points in this thread.
So listen, those molded over rubber plugs are full of god knows. All you can do is HOPE that they're crimped correctly, and most of them seemingly are, but you actually literally never know, and my guess is that they probably don't even test these things for continuity before they leave the factory. And as for the wire? Again, who knows what the insulation is rated to? Or if half the strands are polyester instead of copper? Or the conductor is 12 AWG and covered in mile thick insulation? Over-molded plugs are a cheap, commodity item.
Premium quality, industrial NEMA plugs can cost between $50 and $100 for just the plug, and for good reason. They're really well made and **** near bullet proof. It's not "Just because they can charge more money" and the 6 AWG high temperature wire that's appropriate for the load can cost almost $6 a foot. Zowie! Cheaper than a house though.
Regarding UL listing? I'll betchya that the PLUG that burned up has a nice big fat UL stamp on it, whether it's legit or not is another question entirely. You can buy UL stickers, hologram and everything on Alibaba. And UL listing is nice, but it's no guarantee against a QC problem or especially an error-free install. If Juicebox tried to get UL certification, they wouldn't have been dinged for the plug.
Oh, and a little story about J1772 plugs while we're talking about QC problems. Over the course of 5 years I had a Dostar 32A plug where one of the crimps in to the main power pins came loose. Hard to say how, but after an hour or two, the handle was getting toasty. The wire was so loose that it just popped out of the crimp on the pin. And I just shoved it back in, and soldered it. Cool as a cucumber now, but it could have ended badly for the car, with a nice fire right at the charge port. Kind of makes me think there should be thermistors everywhere, and yeah, no harm in checking screw tightness every once in a while. The only reason I noticed my problem was because I usually grab the handles and plugs every once in a while just for a health checkup. I'd encourage everybody do this once in a while, after the car has been plugged in for an hour or so.
Here's my unsolicited advice for the original poster
Get rid of that outlet and plug. Both. If the plug cooked off, then the outlet took damage too. Either get a good plug and outlet combo, or hardware straight in to the Juicebox, NO WIRE NUTS. Single, continuous wires straight back to the breaker box. (or proper high pressure crimp connections) Properly torque, then recheck after a few cycles, then again after a few weeks, and then whenever you happen to think of it after that. And excellent plan on the smoke alarm. Hopefully it networks in to the house so you can hear it at 3:00 in the morning.
Glad you, your house and your car didn't burn down.