Related: Dangers of Deleting Surveillance Video
I don't recall any VMSes having a UI / easy button / general available function to delete video clips / segments. I am sure someone allows that, but it has to be rare.
And, yes, the reason why VMSes typically make this hard is because of the security risk of deleting video.
Of course, there are ways admins could do it, depending on how video is stored but typically it is made hard, not easy.
Some VMS allow to set the "Recording Days Limit", after that period video is deleted. An untrained integrator may have issues with this, with no exact detail on the System Log ( maybe something like "Recording Options Setup"). It happened to one of my techs with a Nextiva system.
Also VMS may allow to format an entire Drive from the HDD setup menu, thus deleting the whole file system ( maybe an IT expert is able to recover the files).