I was fouled at the start and won my protest — so why was I scored DNS?

Because you still have to sail the course. Rule 28.1 — the string rule — imagines a string tied to your boat along your track: pulled tight, it must touch every mark on the required side. Being fouled, or a starting mark drifting out of position, does not excuse you from a single rounding. Win the protest, miss the proper start, and you still score DNS. (A mark shifting in wind or current is a normal risk of racing under Rule 32.1 — no grounds to abandon.) World Sailing Case 28.

<iframe width="560" height="315" src="https://www.youtube.com/embed/pIWIpFYoOyc" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/start/case-28-sail-the-course-string-rule/">Source: Sailing Kiwi</a></p>

Next on this shelf — The Start