Why is loitering punishable by death?

It's a harsh and unforgiving galaxy where death isn't very permanent. Loitering inside a station blocks the entrance or a landing pad, impeding commerce. Stations live on imports and exports and a blockade would kill residents. The easiest way to clear that blockage and keep commerce flowing is to destroy the ship that's in the way.

I think the game design reason is to help make stations a relatively safe place for "innocent" players, to make it harder to blockade stations, and to give you an automatic escape if you get stuck in some part of the station (like the rack around the slot).

Also, the voice message in stations is pretty funny and really sets a nice "harsh galaxy" vibe.

I don't think I've ever seen an in-world reason for the penalty for loitering. It seems to be designed out-of-game to prevent griefing of the landing areas (by blocking access to them).

The speeding penalties are the same way.

Lore-wise, it's not a death sentence. In fact, nobody ever really dies in Elite: Dangerous. So they're really not killing you, they're just getting you out of the way and making you pay your deductible. Still feels a little harsh, but it's not as bleak as it appears.