The beer can never gets that hot really. It can’t get any hotter than around 212 degrees as that is the boiling point of the liquid inside. Plus the chicken itself is in contact with the sides of the can meaning the surface where the can touches can’t get any hotter than the carcass of the chicken. And in the end, you’re only cooking this in a smoker at around 230 degrees. So, it doesn’t get hot enough to cause any problems. I’ve done many beer can chickens and Cornish hens with no problems at all. The can is on the inside of the chicken and you don’t eat the parts the can touches, and it certainly doesn’t get hot enough to release any fumes.
If you’ve got a Styrofoam cup lying around, fill it with water. Now put a flame (match, candle, lighter) under the Styrofoam cup. You’d think the cup would melt right?
Nope. The water in the cup keeps the Styrofoam at its current temperature. The Styrofoam won’t melt until it reaches a temp of around 460 degrees F, but water boils at 212 and can get no hotter. So, you can hold that flame to the Styrofoam all you like, and it will never melt, but you could eventually boil water.
The same principle applies to the beer can chicken recipe. The can will never get any hotter than the boiling temp of the liquid inside, or the temp of the chicken it is in contact with.