Comment by teiferer

It costs energy.

It's very little energy, but it's literally non-zero, so definitely not "literally zero marginal cost".

Why would the user care if it's negligible? Because very-small-but-nonzero things scale very differently from actual zero things. If the price of injecting something is zero or almost zero, then this gets quickly abused and suddenly your battery drains like crazy because somebody decided that this is an excellent new vector to serve spam. So everybody will deinstall/deactivate this.

And that's why we can't have nice things.