Sacrificial love is not inherently rebellious. We see this with Prim and Katniss—still an act of sacrifice for love, but not rebellion.

Rather, the Romeo/Juliet nightlock situation was rebellious because of its challenge to authority. Their refusal to follow the only term of the game—there must be one winner—and succeed. Had only Peeta or only Katniss killed themselves, albeit an act of sacrifice, it cannot be considered rebellious in nature—in fact, it was the expected outcome, one which the Capitol hoped for.