Agreed, the tablet mount needs to be redesigned to allow for a case. They are not very thick so it wouldn't require much modification, but it is nice to know it is somewhat protected in case for some reason it falls from the mount.
The vast majority of owners use cases for the tablets, smartphones, etc. but it seems engineers never accommodate accessories for this fact. Consider the lightning connectors, they will not accommodate most cases. All it needs is an extra mm of length or so.