This is about spending a few dollars more to have them properly painted.
Think about this, Musk and Bezos are saving a few cents. In exchange of astronomy.
Other satellites are reflective, too. It's difficult to make a satellite completely dark, in part because dark = absorbs heat and it's hard to dump heat in space.
The problem with these internet constellations is that, to work, they have to have thousands of them in each constellation. SpaceX has permission for 12k and is seeking permission for 30k more (https://spacenews.com/spacex-submits-paperwork-for-30000-mor...).
And since when has "It's difficult" been a free pass? Both of those companies beat their chests about how neat their tech is, but when it comes to a genuinely hard problem - as in, how to launch this fleet without fucking up humankind's ability to gaze into the cosmos as we have for our entire history - you're going to let them duck it?
And, to head off the obvious objection: I really, genuinely, truly don't care how difficult it is, why the physics make it difficult, what a vacuum does for heat, etc. I know. It's a hard problem. Maybe you don't get to launch fifty thousand satellites into orbit without solving it.
So, i guess it is as legal as the interference themselves will cause on your 5g phone reception.