Same with private cryptocurrency keys. Otherwise, accessing non-public APIs with weak security by playing around with the URL would be legal.
It's unethical to steal something tangible. Bitcoin has no tangibility whatsoever. You can't steal it.
---
Property is legally defined as 'Not only money and other tangible things of value, but also includes any intangible right considered as a source or element of income or wealth.'
That includes protectable ideas, digital files, financial instruments (like stocks and bonds, loans and credits), computer graphics, certain arrangements of words and quite a bit more.
---
What does tangibility mean to you? That allows you steal the examples in the second paragraph without legal reprecussions nor ethical dilemmas?