Is Bootstrap reinventing HTML?

[nilskp]

I've just started looking at Bootstrap, but there's a couple of things I 
don't get. It appears I have to apply specific Bootstrap classes to get any 
functionality, and those classes cannot be applied through LESS. An example:

On my trial page, I had a disabled button, simply this:

<button disabled>Click me</button>

To have this render, I naturally tried this:

button {
  .btn;
  &:disabled {
    .disabled;
  }
}

This does not work. It appears I MUST apply those classes manually, 
describing to Bootstrap what is already obvious in the HTML

<button class="btn disabled">Click me</button>

Any reason for Bootstrap to reinvent HTML or am I missing some obvious 
switch somewhere?

Thanks!