Where does it say that kneeling is a cost of attacking? All I can find is that it says that you can't attack if you are knelt and that you kneel after being declared an attacker, but I don't see anywhere that it actually states that kneeling is a cost of attacking. If you look at as a restriction and an effect, then I don't see why Black Cells would stop an unknelt character from attacking.

ironchefzod said:

From the Core Rules, under Challenge Step 1: Declare Attackers on page 13

"Then declare attackers by
kneeling any number of your characters that have
the corresponding challenge icon (or are enabled
to participate by some card effect)."

It is the act of kneeling them that declares them as attackers, barring a card that says otherwise.

That is why The Black Cells, by preventing a character from kneeling, prevents a character from attacking (unless a card makes it able to do so, such as saying "does not kneel to attack.")

Since attackers are declared by kneeling the characters, if you cannot kneel the characters, you cannot declare them as attackers. This is not strictly the same thing as a "cost" for attacking, but it is clear that part of the process for declaring a character as an attacker is to change its status from "standing" to "kneeling." Unless you can do that (or are allowed to bypass the requirement by another effect), you cannot complete the process for declaring the attacker.

In a more pointed way of saying it - if I didn't have to kneel my characters to attack, why would I ever kneel my characters to attack?

Since I have to kneel my characters to attack, Black Cells stops characters from attacking (and defending).

ironchefzod said:

The Core set rules states (p. 13) that you "declare attackers by kneeling and number of your characters that have the corresponding challenge icon (or are enabled to participate by some card effect.)" The flow chart for the Challenge phase similarly states "2. Active player kneels attacking characters."

Using the word "cost" is perhaps misleading. Kneeling is, by definition, inherently a part of participating in a challenge (unless a card effect exempts a character from that requirement). Thus, Black Cells, by making it so that a character cannot stand or kneel, prevents that character from being declared as a participant (unless a card effect says he can participate without kneeling; thus BC can't stop Core Set Jaime from participating in a MIL challenge, but he can stop him from participating in an INT challenge).