How can Hades rip out this character's heart?

Question

On Once Upon a Time, hearts are central to many kinds of magic. Rip out someone's heart and you can control them. Put someone's heart in stasis and you can preserve them. The heart of the thing one loves most is a necessary ingredient for the Dark Curse.

It seems to be implied that if one is the product of true love, one's heart cannot be ripped out.

Mr. Gold: I created the curse, dearie, but I didn’t make you. I merely took advantage of what you are – the product of true love. That’s why you’re powerful. And everything you’ve done, you’ve done yourself.

Emma: So you don’t know.

Mr. Gold: Know what?

Emma: Cora… Tried to rip my heart out, but she couldn’t. She was blasted back by something inside me. By… By-

Mr. Gold: By magic. Whatever that was, I didn’t do that. You did.

—Season 2, Episode 9 : Queen of Hearts

How, then, were Regina and Hades able to remove Emma's heart in Season 5, Episode 20?

Answer 1

Because she allowed it to be. When Cora tried to rip out Emma's heart, it was an attack, she was trying to injure Emma in the course of a battle between them. When Regina and Hades did it more recently, it was part of the plan. Emma's entire reason for going to the Underworld in the first place was to split her heart in two and give half to Hook, as her mother had done for her father previously. She wanted her heart taken out and split. Later, she'd already been told her heart would need to be weighted or judged in order for Hook to get to the Ambrosia, so again she wanted it taken out. While Hades did it rather suddenly and without warning, it was still what Emma wanted.

Whatever magic Emma produced to keep Cora's attempt from succeeding can be thought of as more of an instinctual response to an attack, rather than something Emma did consciously (at the time, Emma didn't even know she had magic). But when Hades & Regina did it, it was with her (at least tacit) approval.

An analogy might be instinctively taking your hand off a stove's burner vs allowing your cold hands to be warmed next to a campfire. One is an instinctive reaction to danger, the other is a welcomed action, even though both result in the same event.

Answer 2

Hades is a god so I would assume that he is more powerful then Cora. It is possible that he overpowered what ever defenses she had over her heart.

Also I took the conversation between Emma and Gold to mean that those that are the product of true love have the ability to do magic, not that they can't have their heart removed. When Cora tried to pull her heart out, her subconscious mind instinctively did a spell that protected her heart. Since this was done on instinct not on purpose then that means that she might not have learned to activate at will this the spell that protects her heart. Making it possible for hades to take her heart.

Lastly it is possible that the writers just forgot about this little rule.