Maintenance for the week of August 3:
• PC/Mac: No maintenance – August 3
• ESO Store and Account System for maintenance – August 4, 5:00AM EDT (9:00 UTC) - 4:00PM EDT (20:00 UTC)
It's probably for account security reasons. They don't want getting someone getting either of their accounts hacked and then being able to unlink easily. It likely gives them a chance to double check the account for suspicious activity before unlinking to ensure it isn't a hijacked account.