Full disclosure: I am also a lawyer, and I work in markets trading at a bank. That's not an endorsement of Coin Center's conclusions, but it's my informed opinion their arguments are sounder than the idle speculation of the author.
If there's anything that Ether is similar to, it's a currencies (my area), collectibles, or commodities (adjacent to my area). There's no underlying business that's being invested in or paying dividends to you. It's 100% a speculation on appreciation.
I'll give you an example. Say Vitalik owned a oil exploration company, and he also owned oil rights to a big patch of oil in Texas somewhere. Instead of selling shares in his company to raise money, he pre-sells the oil. "You give me money for the oil, and I'll go dig it up for you."
You don't own shares in his company. You own oil. Oil isn't a security.