I was riding arvak trying out the new mounted combat to find that I could wield my 2 handed sward with 1 hand.
I then took out my daedric warhammer and wielded that with 1 hand whilst on horse back.
If the dragonborn could do this all along then why can't we dual wield daedric warhammers

It's probably so Bethesda can recycle an animation and keep people who use Two-Handed builds from complaining that they're being discriminated against, though why they don't let mages cast spells on horseback (or use their staffs/scrolls at the very least) is still anyone's guess.

it's so one hand can hold onto the horse

But that would imply that they can hold it one handed when not on a horse.

This makes me picture the Dragonborn playing polo.

You can but it would be highly ineffective...why would you hold it with one hand when you can use both hands? If you didn't already notice, your swing speed while on a horse is much slower, not to mention you're sitting down.

"Oh yea time to one handed style on mah horse YEEEEEAAAAAAAAAAAH!"

*arm falls off*

"DANGIT I FORGOT TO SAVE!"

"Pulls out two handed daedric warhammer, attempts to hold it with one hand... falls off horse"