Sion is part of the Noxian Reformists, but he is still "friends" for Katarina.

sion is also a cat person

Well, Sion strikes me as having two personalities. He has his normal personality, when he's in battle, where he's fanatically loyal to Noxus and just wants to destroy and kill.

Stuff like: "NO CHAINS? FREE! KILL!

IN REACH. YES! DIE! DIE!"

But at the end of his Lore he mentions this:

"So dark. This pit. I remember. Grand General. His whispering. What was it?

Not who I think.

Fading. Mustn't forget.

Message. Cut. Remember.

SION – Beware ravens."

When Sion is lucid again, he seems to know that Swain can't be trusted, someone has warned him about this. Someone else in this "pit". In my personal Lore dream world, it's Marcus DuCouteau, but I don't know if Riot's still working with that character.

So perhaps one of his personalities is more sympathetic to old Noxus like Kata/Talon. He is about the oldest you can get, being alive when Jarvan's great-grandpa was king.

Why isnt Sion allowed to like Katarina as a friend just because they are on different sides?

Obviously they dont get to hang out or some crap, but mutual respect isnt hard to believe, especially when it seems he doesnt give a shit about Swain.

There's always the possibility that they're not actually allies in the new continuity and Riot simply hasn't updated their relationships to reflect that Kat was no longer responsible for recovering Sion's remains.

Actually looking at his champion page,he doesn't list katarina as a friend anymore. Instead he lists jarvan as an enemy and swain as an ally and an enemy(reflecting how one of his sides is totally loyal to swain,while the other one seems to despise him)

Sion knows that swain is not to be trusted and katarina hates swain since the Du Coteau´s family name got disgraced by him when he reached his power peak as Noxus Commander / Grand-General.

My theory is that Marcus Du Coteau was presente or was sacrificed alongside with the Jarvan (not the Jarvan IV) blood to ressurect Sion, possibly implying that Marcus shared the same blood as the Jarvans bloodline from Demacia, this means they could have the same father but diferente mothers since its quiet common to high rank nobles to have affairs outsider of their marriage.

just my 2cents