Instagram has started working according to the new set of rules, disabling accounts with specific number of violations within specific period of time, so maybe it's the case here.
I have reported this account couple of months ago, and Instagram's action was (unexpectedly) quick. Next time I've reported it for the same stealing behavior, Instagram gave me a little more trouble, but still deleted the content I've reported eventually.
It seems to be more difficult for the copyright holder to prove his/her case than it is for the shady instagram account owner to steal the photos, but copyright owners can prevail in the end (like in this case)...