I propose that it should use a Baysian prior where the background knowledge is assumed to be an A.
While it may be true that most ToS are onerous, suppose we look at a ToS document as a collection of terms of service. It's only the terms of service that cause a removal of rights that would otherwise be assumed. The more terms there are, and the more onerous each one is, the more rights can be removed. But before there are any terms, no rights are removed, so that situation should be an A. Diminished from there, depending on how many terms there are, and each one's onerousness.
While it may be true that most ToS are onerous, suppose we look at a ToS document as a collection of terms of service. It's only the terms of service that cause a removal of rights that would otherwise be assumed. The more terms there are, and the more onerous each one is, the more rights can be removed. But before there are any terms, no rights are removed, so that situation should be an A. Diminished from there, depending on how many terms there are, and each one's onerousness.