why can Ammumu throw his q, while grounding?

i played cassio, in a aram game, my enemy was a ammumu and this little guy carried this game, only chause he can't be grounded, why is this a thing? If you are grounded you can't dash! the same thing happend with Singed's W, WHY! i this a thing, why can he use his q! while grounded i don'T get it {{sticker:slayer-jinx-unamused}}
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