They are "free" but with significant strings attached. The key thing about Splashbase is the images have almost zero strings attached. I say almost because even public domain images will have caveats; for instance, one cannot make it appear that someone is endorsing a product when they did no such thing, etc.
Right, it doesn't search sub-categories, so while not completely useless, it's merely next to useless (no efficient way to search the corpus of public domain images on Wikimedia Commons).
The main issue with pointing to Wikimedia is that it seems to funnel people into Creative Commons licenses. Maybe that is not the intent, but that is what happens. I like the idea of a prominent place that encourages the Public Domain.
Yeah, I recently had to make something commercial where I didn't think attribution was going to be possible, and I knew that SA wasn't going to fly. I figured I'd check out Wikimedia Commons but found it very hard to find truly public domain images. Though there might have been usable pictures available, I ended up just going out and taking one myself because the search wasn't really practical.
I also think a lot of people of the current generation probably don't think that much about licensing and copyright and don't realize how many strings they're attaching to the use of their image when they upload it, when my guess is that for most people, one of the CC licenses like CC-by-SA is essentially the same as releasing it into the public domain (for them), but of course it excludes it from inclusion into any non-CC works for likely over 100 years.
https://commons.wikimedia.org/wiki/Main_Page