My WAG is that if you’re using a gas stove and the flames are wrapping up around the side of it, you could melt some of the plastic on top.
you’re making tea - its supposed to be relaxing - don’t rush.
Its more of a philisophical thing -
Actually it’s usually for coffee and I want it FAST!
We have a set of aluminum non-stick pans that had the same warning.
I once accidentally boiled an enamel tea kettle dry on high heat. Oops. Had to get a new element, and was lucky I didn’t burn the damn house down.