As many others here have surmised, the default license for data, photos, and sounds is CC-BY-NC because that’s the least restrictive license that still prevents one’s content from being used commercially without their approval - an assumption I think most people have when they sign up for iNat. Personally I like it that way - I’m happy for anyone to use my photos (and I spend a good amount of money on photo equipment and have sold some photos commercially) for non-commercial purposes, but I wouldn’t want them to be used for commercial purposes without my knowledge and approval. I should also note that at least one Flickr image of mine was used commercially without my approval (and was even misidentified by the user…grrr…). I’m bummed my CC-BY-NC license means my photos can’t be easily used on Wikipedia, but for me CC-BY-NC is the best fit.
I’m not sure how many people have taught others how to use iNaturalist, but even getting a class of people to download the app, sign up for an account and start observing presents numerous hurdles and can take much, much, longer than you’d think it would. I would not be for a dense explication of licensing options for three separate types of content when a user signs up for iNaturalist. The idea is to get people out and observing, and I think onboarding should be focused on using the app/site correctly.
I do like @astra_the_dragon’s suggestion of some sort of outreach after a user has posted 100 observations or a similar number and they have the hang of iNat basics. This will reach an audience of more informed users who understand iNat better and can make a more informed decision.