I hope this anecdote is intended to demonstrate the unreasonableness of the average person, rather than the unreasonableness of firefighters.

A fire that can be put out by an extinguisher doesn't stay a fire that can be put out by an extinguisher. The same for suspicious smoldering in a wall.

If you want to deal with a fire in a calm manner yourself, then deal with it yourself, right? When you call the fire department, you've declared that there is a situation bigger than you can handle. It's only reasonable for them to assume it's growing.

The only downside is a few extra truck rolls.