Is it really ultrasound? Microphones on phones may be able to pick this but what speakers are able to play even 20 kHz? And these apps should always listen, that will drain battery quickly, so these apps will be listed in top power-consuming apps and user will notice it.
your first point coupled with diminishing free air range in bands above 40kHz makes this hack seem like a futile attempt to simply gather as much data as possible, as low or uncorrelated as it might be. Some phones might be more cappable than others, however, and air is dense enough for several meters of significant ultrasound transmission at low power. With e.g. Samsung TVs always listening, I wouldn't be surprised if they optimized the speakers and micros in that regards, while benign applications aren't out of the question either.
The second point however is an overly broad generalisation disregarding the technical inaptitude of the target group of such apps (wherein I might include myself).