Tax Code and Unit Price didn't clear when we change items at sales invoice

new screen

choose non inventory item which set tax code

when remove item
unit price and tax code didn’t clear

This is normal. You did not clear the entire line. You only cleared the item. The account cleared because it is hard-coded with the item. Description, quantity, unit price, etc. are not.

It would be great, if system clear those value

Look at things from a wider perspective. You would not be pleased if the program deleted things you had not chosen for deletion. Suppose you had entered a special discount and quantities for an item, then realized you had mistakenly chosen the next item in the dropdown list when originally making your selection. Why should the program delete your quantity and discount entries that are not related to the inventory item itself?