You don't remember what happened after moltdown coin refund? Insanity, people one shooting eachother in bgs, due to everyone having best weapons but no armor, arenas was a mess, first ds hc madness kill was done with everyone already having hc madness items.
Yeah you could argue "release the items in the store only after a month or two has passed after raid release" but even wotlk and tbc people complain that there is not enough time to properly do the bosses, not everyone can raid bwd with 346 ilvl items, lower end raiders would suffer a lot and neltharion lacks "casuals" who don't want to rush and just enjoy the game.
Lastly, immediately after ds hc items get released EVERYONE will get them, no one will spend coins on bwd or bot items, only best in slot matters and when people have few thousands coins it is really easy to understand that there will be a problem if such approach is taken. My solutions would be: don't give coins back OR have a cap on returned coins OR modify shop not to include heroic difficulty items(by this i mean Firelands and DS)... Feel free to flame my stupid ideas