Ekko should be able to use R when he is rooted because when you are rooted you can't move but you can use your spells and he doesn't move he travels back in time using the device on him. I get it that you shouldn't be able to R when you are stunned, charmed, polymorphed and you should only be able to R when rooted.
Roots are preventing the use of all movement spells in this game (dashes, blinks, etc.). Since Ekko is considered a movement spell, you can't activate it.
As mastery 7 Ekko myself, I think it is ok that it works like that. With his ult and E for movement plus W and Q for slowing, it's hard enough to catch Ekko already. Let's not make it even harder.
