Why does Electrodominance work with sorcery speed permanents at instant speed, while Idol of Endurance does not? I understand that the rules are listed on the Gatherer, but I want to know why these rulings aren't considered inconsistent.
Because Idol of Endurance provides a duration in which you can cast the spell. It says "Until end of turn". Since it provides that duration, you don't get to bypass the normal timing for casting the spell.
Electrodominance on the other hand says nothing about when the spell can be cast; just that you can cast one. This means the spell is cast during the resolution of the spell. By doing so, it provides an additional permission to cast the spell for you to cast "right now". So, you can cast the spell it says to cast.
It is especially important to recognize that without the necessary rules to allow you to cast a spell during the resolution of ED, you couldn't cast anything (not even an Instant). The rules do not provide permission to cast spells during the resolution of other objects unless said objects provides the permission. Rule 608.2f:
608.2f. If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a-i, except no player receives priority after it's cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.
WizardMN's answer is correct (as usual).
However, I do not see how the rules (relevant rules are 608, 610 & 611), as they are written, actully allow anyone to reach this conclusion.
I know we've had that discussion before, but it still hasn't been addressed properly, IMHO.
Electrodominance on the other hand says nothing about when the spell can be cast; just that you can cast one. This means the spell is cast during the resolution of the spell. By doing so, it provides an additional permission to cast the spell for you to cast "right now". So, you can cast the spell it says to cast.
It is especially important to recognize that without the necessary rules to allow you to cast a spell during the resolution of ED, you couldn't cast anything (not even an Instant). The rules do not provide permission to cast spells during the resolution of other objects unless said objects provides the permission. Rule 608.2f:
608.2f. If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a-i, except no player receives priority after it's cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.
However, I do not see how the rules (relevant rules are 608, 610 & 611), as they are written, actully allow anyone to reach this conclusion.
I know we've had that discussion before, but it still hasn't been addressed properly, IMHO.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules