I think that was a bug. The property's backing item is named "PageCategory" and it seems that we don't properly map from property name to backing item name in the EditAttributes helper. Using @Html.EditAttributes("PageCategory") should be a workaround.
I will investigate and report a bug. Thanks for pointing out.
I see no reason why this should not work and have created the following bug to handle it:
Bug #97602: Edit hints for the category property does not work
It works on page, but is it possible to show editable Categories for shared block in On-Page Edit Mode?
I just wanted to confirm whether it's by design that the EditAttributes HTML helper method doesn't work for CategoryList properties in EPiServer 7?
In the markup I can see that it has added attributes such as data-epi-property-name="Category" and data-epi-use-mvc="True", but I don't get an overlay.