Hi,
That's not the case.
I just tested and if you change the email address of the user via the Joomla user manager of Joomla, the email address is also changed in HikaShop.
There are two cases when this would not happen:
1. You unpublished the HikaShop user synchronization plugin, which, among other things is there to handle this.
2. You're updating the Joomla user email address by another mean which doesn't call the onAfterStoreUser trigger of Joomla as it should.
Using the mass action system to do such synchronization is not possible, but anyway, you should not normally have to as neither of these two cases should happen.