This is a question we get asked a lot, but when you think about it, it does make sense.
By not allowing you guys to permanently remove who's in your lists or who you've sent campaigns to, we provide you with an audit trail that you can use to protect yourself against a complaint.
If a subscriber complains about not being removed from one of your lists, you spamming them, etc, there is always a way to confirm the date they subscribed to your list and when they were removed from it. As long as you're following best practice and have obtained permission, you're covered.
Of course, you can always set a subscriber as inactive if you don't want them to receive any campaigns or hide any test campaigns from your clients with a single click.