why jump pad forbids double jump while healing

Certainly due to balance concerns. If you couldn't perform a regular jump while using meds, that would be a big nerf to meds. It would be impractical to instead remove all waist high barriers to circumvent such a nerf.

If a double jump was possible while using meds, it would be an overwhelming and very annoying buff to meds, not to mention the already incredible team utility that Octane pads offer.

Nobody wants to chase someone who can fully reset while travelling at the same speed and distance as their pursuer AND change trajectory in midair.

Pads are already great offensively, defensively, even able to exfiltrate knocked players in the direction of their choice.

You always have the option of popping heals barely after the double jump.