The tag isn't just for categorization of the question, it's also to help others find the question through the RSS feed and the like.
Tagging it C# ensures that a large cross-section of people that use the .NET Framework daily will see your question. They may not be following the
.NET tag (partially because it applies to every C# and every VB.NET question out there), but they will follow the
C# tag, which means more eyeballs for your problems, and more chances of an answer.
It's not accurate to do that, but it's useful. In time, I would expect it to be edited out, but in that golden time before you have an answer, it's nice to get as many eyeballs on it as possible.