What's the logic behind Contribution's Payment Method for offline Contributions?
The reason is probably that no financial transaction (which is different from contributions) is recorded for pending contributions. 

Once you have a financial transaction e.g. if you change the status to completetes you will be able to edit it in the edit contribution screen (click on the little pen - see screenshot). 

I think this behaviour was introduced only recently and I am not sure if it is a bug or a feature that you cannot change the payment instrument before you have a financial transaction.