That's not true. There is no Default. The assumption is that there are things you buy or sell or buy and sell; but any item is linkable to nearly any account.
It is true
Default = means the way it is set up to start with when you create a NEW item.
Sure you can modify it - modify is not an attribute for default.
Service asks for income, non inventory asks for expense - no one said you can not select only those kinds of accounts
Other Charge Item Type for Freight or Restocking Fees.
Yet you just said any account can be used on any item, so there is no set use for any item - those are your definitions