You know what a bunch of those have in common? A ton of effing safety measures.
Chainsaw chaps, ROPS, etc all exist exactly because those things are so dangerous. The power density of an e-bike is at least one if not more orders of magnitude less than a 40kWh bank of batteries.
He also has the potential to not only burn down his house but kill a lineman working on the line if the power ever goes out.
With all of those things, like batteries, it's not good to assume the general public can use them, so they shouldn't be as common as elevators, shopping carts, and other ubiquitous tools/machines. But it's very dis-empowering to claim only elites can use these tools.
Your final comment regarding lineman safety is completely decoupled from DIY battery pack with re-used cells. That would be a failure of the inverter in a grid-tied system. You could have an unsafe grid-tied inverter with a 100% functional battery pack with every certification in the universe and backfeed the utility lines, or I could connect a faulty battery to a proper grid-tied inverter and it would not cause the utility lines to be energized by my house when the utility shuts off the power upstream.
Additionally, you can use a home storage battery without a grid-tie inverter at all.
It's not that they can't be made perfectly safe, but that you take reasonable safety measures and mitigate risks appropriately. If he had this off in a shed ~40ft from the house that's a lot different than inside the garage.
The article didn't mention how he's connecting it to the grid so there's a very real possibility that he could have an inverter directly connected which is why we have building codes and inspections to catch things that have the possibility to kill people or do significant property damage.
I don't see why it's a crime to build a battery pack if you aren't working for Tesla.
I still don't get how you can bring up mis-wired inverter as anything related to a DIY battery pack. Without using any batteries, I can go take a wire strippers and 2 extension cords, make a male-male 120VAC cable, plug it into the gas generator in my shed and plug it into an outlet in my house when the powers out and shock a lineman.
In the country I live in we have building codes that describe how the work must be done. An owner without a license is allowed to perform that work and have it inspected and permitted. Screwdrivers aren't like lightsabers that can only work if you're a jedi (or have a college degree or certification).
I disagree with lumping every form of DIY together. Incompetent DIY projects are dangerous therefore all DIY projects are dangerous?
Yup, and that's highly illegal because it can kill someone. If you want to do that you're required by code + law to have a mains transfer switch.
I don't think anyone is saying you can't play with battery packs, it's just that this guy is a freaking idiot for doing it in his house where any miscalculation can cause a chain reaction that may kill him and his family.
It's about orders of magnitude and safety mitigations.
Look at blackpowder/low power explosive regulations. You can keep a small quantity in your house to reload/etc. However if you want to store more you need a separate shed + BATF is allowed to show up at your house and inspect it any time. Back when I was involved with amateur rocketry all of the our vendors went through this process. It's a standard thing when you're involved with elements that have a high potential energy and can go wrong if not treated with the right respect.
Which part? Powering your house w/out transfer switch (when disconnected to the utility), or actually back-feeding power to the utility? I would be surprised if the former was uniformly illegal across the US, but the latter certainly is.
But it's a whole different kind of stupid to recommend building your own table saw with salvaged parts and zero safety features.