I think it would help if you asked your question again, or clarify the subject of this thread with more than just a question mark.
No one ever said you can't kill yourself. Someone might have suggested it's not always a good idea, but go ahead and do it if you don't have a hog. If you feel so strongly about it, why not?
Are you asking "why is walking allowed on Death Island"? Do you think we should have a similar rule for Death Island like what we have for Infinity? Infinity = no walking away from the base, and Death Island should also = no walking away from the base?
Are you suggesting we have a rule that, if there are no hogs at the base on Death Island, players cannot walk away? They can only leave by hog or suicide?
I'm trying to understand your argument, and that's the best I've come up with.
Also, in my personal and practical experience, I've intentionally killed myself before and respawned in the same place. Did it again, and respawned there again. That's two 15-second penalties. 30+ seconds and I didn't go anywhere on the map. You're very right that a 15-second penalty is faster than walking to the other side, but there is no guarantee you'll spawn at the other base. But I obviously don't have your good luck when it comes to killing myself.
I will always choose to walk if there are no hogs at a base. It would seem to be better for the team too, because while I'm waiting out a suicide penalty, I could have been picked up by a teammate and helped defend them, or hijacked the hog of an enemy while walking to the nav. Or hey, maybe I'll get killed and spawn at the other base, so many possibilities!
The only guarantee you get from suicide is a 15-second penalty. We'll always allow and encourage people to use that 15-seconds better.