Lots of people have different accounts that they use. Sometimes I use a junk account to view webinars that require an invitation. Other times I just want to be able to copy meetings from my personal account (or a second business I have a different email address for) to ONE calendar, so I don't forget meetings, and I can easily see them all on one calendar.
If there's a true "problem" (something that could cause damage to files or privacy) then I understand needing to make a fix. But I think Microsoft doesn't understand how many people DO commonly like to copy meetings from different calendars to one they use a their MAIN Calendar.
And by just unexpectedly turning off this feature, it's really frustrating. Please fix whatever issue you are concerned about, but allow people to continue copying from one calendar to another.