Emission standards

I own a 5-speed automatic '98 323ti. I've noticed that when the engine's
it will shift gears much later then when warm. Also the lock-up will
come later the colder it gets. Both pretty logical but I wonder what the
real reason behind this is.

My previous car ('94 318ti) was a stick shift. The manual stated clearly
that revs should stay limited to 2000 max when cold. The automatic revs
up way beyond 2500 when cold.

I suspect that emission laws play part in this as well.

Any thoughts on this?