I want to make a quick point to update my experiences from the past few hours, all of which are great.
*I was interdicted by an eagle who demanded my cargo. I scanned him, saw that his record was clean, and held fire. I was carrying nothing, and a moment later he said "no cargo? waste of time." He took off, I went on my way.
*I was interdicted again with cargo in my hold. Same thing happened; the pirate's record was clean, and I held fire. I waited for him to start shooting at me, giving him a "WANTED" designation, and wrecked him hard. I got a bounty voucher.
*Got interdicted a third time, scanned, saw the pirate was wanted, opened fire before he did, got a bounty voucher again.
Long story short, SCAN THEM. If they're wanted, you're all good. If not, wait for it. I haven't had a problem so far following this simple method. A straightforward game mechanic to be sure, though it might not be intuitive as has been said.
The problem is the 4th situation, one you apparently never had. They're clean, they ask for cargo, then attack you first but just happen to miss every shot. If you land the first shot, you're considered the assaulter, no matter who actually started firing on who.
Thankfully this has already been declared a known issue.