Apple has whole departments that enforce the uniformity of User Experience across the way Applications work. So it was puzzling to me at first why these should behave differently from, say Cut and Paste in a Word Processor.
On closer examination, you generally are not selecting and cutting the middle six letters, or pasting with an insertion point into the middle of a field of letters. You are instead manipulating these email Addresses as "objects", and the drag-and-drop metaphor can be used to duplicate and re-use them.
If you were to instead take only the firstname out of an Address (which is difficult, as mail wants to select the email Address as an "object" in its entirety), and paste it into the middle of a different field that is actually text, I think the right-click options should work as you expected.
No offense taken. This forum format does not lend itself to subtlety in expression.