What is now C.R. 725.6 was added to the comprehensive rules with Shards of Alara, and according to the September 2008 update bulletin, it was added precisely "to cover a corner case involving the Shards of Alara card Prince of Thralls (and probably some other cases)".
What boggles my mind is rule 722.6, which states a somewhat arbitrary exception to rule 722.3.
I'd be very appreciative if anyone could provide some examples illustrating how it actually differs from what is said by rule 722.3.
Specifically, how do the words 'if no player chooses to perform [B]' (722.6)
actually allow a player to escape 'must then make a different game choice' (722.3)?
Does it solely rely on the fact that the effect actually contains the word 'unless'?
Any volunteer?
What is now C.R. 725.6 applies only to effects of the form "[A] unless [B]" (an example is Prince of Thralls), and none of the cards given in the scenario of comment 1 contain that form, so that rule doesn't apply to this scenario. See also this thread.
EDIT (Apr. 21, 2022): Update rule numbering.
EDIT (Apr. 21, 2022): Update rule numbering.