Why does Kaisa's legend ability have reaction speed if activated abilities can only be used during your turn when the chain is empty?

Ruling: Activated abilities with reaction speed can be used in any reaction speed window, not just during your turn when the chain is empty. The reaction speed designation specifically allows the ability to be used at times beyond the default restriction.

Nuances:

• The default rule for activated abilities is that they can only be used during your turn when the chain is empty
• When an activated ability has reaction speed specified, it overrides this default restriction and allows use in any reaction speed window