I agree with you that you should be able to charge whenever you want, but something to consider is that by paying for supercharging, you're subsidizing the creation of newer superchargers down the line and also maintenance of existing superchargers, which not only lets you continue your local supercharging, but gives you more options when traveling.
You shouldn't expect to get $2500 of electricity. You should expect to get some portion of that in electricity, and the rest going toward maintenance, new locations, and research for new charging technology. All of those benefit you and you shouldn't expect to get that for free.