All workflows are managed by JIRA, but I guess what you mean is that the box is not there when you are not using "simplified workflow".
The reason is actually the other way around. The box is there because the workflow is simplified. Simplified workflows are actually just JIRA workflows that are built by JIRA Software based on a very plain set of rules - which boil down to "a workflow is a list of status and there are transitions from every status to every other status". The reason the box is there is because JIRA (not so much JIRA Software, but JIRA Core) relies on the resolution to know when an issue is closed, and hence the system needs to know when you want it set or blank.
For a non-simplified workflow, the person building the workflow is responsible for handling it, and you don't want the project admins messing it up by setting the resolution at the wrong time.
Please, it's not "resolution status". Status is an indication of the position in the workflow. Resolution is a field on the issue. They are totally separate (technically), and in JIRA-speak "resolution status" is misleading because we don't know which one you mean.
You can't set anything in validators (or at least, shouldn't), they return a simple pass or fail flag to allow or prevent the transition after the user has committed it.
What you're looking for is the post-functions - they can change issue data. There is one for "set field value" in JIRA off-the-shelf - that can be used to set resolution. It can also clear it - when you select resolution, you can set it to <none> as well.
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