What's the popcorn button in the microwave for if every pack I buy says to not use it on the instructions label?

Because the popcorn catches on fire . Personal experience. So dumb why there is that button. All the popcorn was black like charcoal

It means the microwave is out to get ya.

Go figure ! Just ignore the button (that should be easy),and cook according to popcorn package instructions.

Use it the first time and if you get lucky it will turn out to be good. If not add more time until it is cooked. Companies like to make equipment look easy and foolproof by adding these presets for inexperienced cooks.

In reality cooking is more complicated.

The button is there for the same reason the automobile speedometer goes up to 140 mph when the car can only go 104mph. Non-verbal selling point. 

In the case of the popcorn button warning it should really say use the popcorn button but MONITOR the popping progress anyway.  Not sure how sensor cook popcorn works but my guess is not very well. 

I use it all the time, but then my microwave "sensor cooks" turns out fine.  But people who buy cheap microwave ovens have to stand there and wait for the popping to slow down.

It is there to make the number of buttons even.  If it was not there they would have to reconfigure the entire control panel and that would be expensive. 

The popcorn button probably cooks it too long.  I think I would give it a try.  Just stand by the microwave.  If it stops popping while you still have time on the oven then take it out early.  Enjoy!