Even being suffocated by brainwashing and told all she’s good for is studying, the pen brings back her connection to Usagi and Usagi’s love and support for her.
The pen is what unlocks Ami’s power but it also is a symbol for her bond with Usagi. That’s because her bond with Usagi is her power. It’s Usagi’s support of Ami that unlocked Ami’s true self. It’s not just Ami’s intelligence that makes her a hero, she needed that friendship to unlock her own power. She can’t use her intelligence properly if she’s alone and has no connection to anyone, nothing to give her hope and confidence. Usagi showed her how much stronger she is when she focuses her intellect to help those she loves and wants to inspire and protect. It was the bond of love between them that showed her how strong she really can be, and how much power she has.
So of course she chases after it no matter what. All the brainwashing in the world can’t seperate her from those feelings, especially with Usagi there to back her up and once again show her support in her time of need.