It seems unlikely that they really do care about a handful of DIY people, or that there is much they can do about it.
I imagine the larger concern is the army of startup companies that could start producing EVSEs at very low cost once it becomes widely known just how simple they really are and demand starts to ramp up. None of these companies really have a decent barrier to entry or any significant intellectual property so they need to create a protection guild and lobby like crazy to maintain position.
Maintaining the veil of technology sophistication is an important part of that. Videos on the web that show you can build one in your garage in an afternoon really don't help at all.