When someone puts a Pokémon on the GTS they know what they are doing and taking full responsibility for it. If you offered a Pokémon and they accepted it means both people are fine with the trade.
Also I do give out Shiny legends for good Pokémon and occasionally give away shinys/legends/shiny legends as giveaways.
However some people do shiny trade. There will always be people who want to trade shiny for shiny.
So, to answer you. Do people in the GTS always expect shiny Pokémon in return:
No. No they don’t.
That’s not saying that all people will/won’t want shinies for shinies.