Eavesdropping is borderline, but it’s outright unacceptable for the eavesdropper to interject.
Also, people need to have the sensibility to know which conversations they shouldn’t be having publicly. I was at a coffee shop recently and a man was sitting there doing a video-call date on high volume. These two adults we’re making really pathetic/inappropriate child banter on their call, while in the middle of a crowded coffee shop.