The UL is a certification program, They can't really tell you what you can/can't install in your own home. They can't even tell you what you can/can't sell to someone else. The issue is that most distributors won't buy/sell things w/o UL approval (for good reason). I'd most certainly guess that the creators have had them in their home for some time.
When I worked in commercial electronics, it was very typical for engineers to install development products in their home, long before UL approval.
Depends; it would be reasonable for a dense city to still have restrictions since fire has a propensity to spread and if your place burns down, your neighbors' might too.
Does that only count for wiring that is attached to a public grid?
E.g. if I have a homebrew energy system that doesn't touch a grid, am I free to attach anything I want including such USB outlets?