The answer is in your question.
I try this when login as admin and editor.
The roles have the
unfiltered_html capability that allows them to put whatever HTML they choose, including
<script> tags, where ever they choose.
Is is a security risk? Only if you give folks you don't trust admin and/or editor roles. Or someone gains access to your an admin/editor account. Or there's another security hole somewhere in the core that allows privilege escalation from a lower to higher user level (unlikely).
By itself, it's not a security risk. Admin and editors need to be able to do things to actually manage the site.