House of sorrow and Creepshow Cassino - when is an action announced?

Yomyael wrote:

Thank you for the ruling and the detailed explanation. However, I still find this weird, considering the following things:

a) For the "as a card is played" window, these things behave quite differently. Could you give a reasoning to this difference?

The list of cancellation effects is kept very small on purpose.

Yomyael wrote:

b) In most cases, playing a card and using a card already in play are considered to be the same things (regarding impulse and other restrictions). However, here they are different, as you can use effects but cannot play cards. Could you also give a reasoning to this difference?

The timing for playing cards is usually very specific, either explicitly ("only usable during...") or implicitly (e.g., master cards, stealth cards).
The timing for using effects (e.g., the Barrens) can be noticeably broader by design.