Just what does being installed by a DIY have to do with anything?Because the heater should be tested and listed with factory connections and a heat resistent chamber where the switch box might be all DIY installed.
I am tired of having to design and install more things every three years around the DIY mistakes (not even mistakes - usually just ignorance) made in the past. We have many stupid rules that are because of people that stuck their noses where they don't belong.
Wall boxes are intended to contain splices and devices - which have a risk of developing heat because of a poor connection. If the risk of developing heat in those situations were not that great we may not even require boxes where junctions happen. If a DIY doesn't install something correctly and burns his house down why should the entire electrical industry have to change their ways to accomodate the DIY that still will not do things right even after the changes?