Because interference is traffic congestion, not me ripping the road out of the ground and hauling it away on a truck. If a frequency is busy, you just get noise and reduced range. The difference is that you don't have as many independent protocols running through the air as you do on the streets.

You can still use it, but your usage is diminished. On the electric grid, there is a peak voltage the line can feed to all recipients before it is maxed out and can't provide more juice. If I am on the grid with you, I'm diminishing the maximum power you can take off a sufficiently strained grid. It isn't stealing from you to use it though, and in the case of power there is very little litigation about people consuming too much electricity (from what I know).

There's no litigation over taking too much power from the grid because the grid is clearly owned by the power company, and they get to tell you what you're allowed to do with it (within the limits imposed by regulations). If the grid were simply a free-for-all the way you're proposing radio spectrum should be, things would be quite different.