If some coin (include bitcoin) provide only (presume) desktop wallet, then anybody can use that computer's wallet's coin?
No.
Just run that wallet program.
Which will allow you to SEE the coins, but not SPEND them.
Because there is no login process at that desktop wallet.
My desktop computer requires me to log in. If you don't have a password on your computer, then that is YOUR FAULT.
Then, my wallet program is protected with a different password. If you did not set a password on your wallet program, then that is also YOUR FAULT.
And owner also doesn't know his secret key?
No. As long as you create a backup of your wallet, there is no need to know your secret keys.