Apple permanently closed my account without reason and kept my balance.

Read section VII part B of Legal - iCloud - Apple

You can also browse the rest of the article and see if any of it provides an indication as to why the account was closed, though the things they mention are pretty broad and vague, such as abuse. From what others have posted in similar situations, Apple won't tell you why, just the status of the account.

Since it is not clear exactly what triggers account closure, I personally would avoid letting personal account balance build up to high amount.

They have cause, but are not required to reveal it to you. I would suspect you would know what that is and if you don't refer to the linked article for a list of violations where Apple would close your account.

• Apple may at any time, under certain circumstances and without prior notice, immediately terminate or suspend all or a portion of your Account and/or access to the Service.

Legal - iCloud - Apple

You will know better than anyone here precisely what term(s) you violated.