Don’t know for sure if this is the reason since it’s not explained, but it’s definitely clear that this is by design (is the same way for all searches AFAIK, and for some pages you can see that the filter is applied after the unfiltered query loads first) so it can be assumed that it’s so you can’t find unfiltered content anywhere on the website. Someone could search for something inappropriate and then make a screenshot of it and say “Look how inappropriate this website is for kids!”.