It's like the old joke about the rookie cop who's too nervous to investigate a big spooky warehouse at night. He shouts into the darkness "is anybody here?"
A few seconds later, he hears a voice say "......no". So he relaxes with a sigh, and goes to report the all clear to his superior.
Off the cuff, a better user experience altogether might be to prompt attendees afterwards to submit their biggest take-away, instead of a patronizing poke.
That way, not only does it confirm paying attention, but you can also get a signal of course effectiveness.
If I had to guess, it's because the programmer couldn't find out (soon enough) how to do another kind of dialogue box and then went with the standard yes/no but then didn't look up how to handle "no" and assumed everyone would do "yes".
If I had to guess it's probably because not everyone attending the webinar needs CLE credits reported. I can imagine people intentionally selecting no if they're not an attorney or if they're licensed in a jurisdiction that doesn't require it.
Or perhaps it's to teach the most valuable CLE of all - too hasty a response risks a critical mistake. I can't even recall the specific topic of the webinar but I definitely learned a lesson!
A few seconds later, he hears a voice say "......no". So he relaxes with a sigh, and goes to report the all clear to his superior.