In many ways, the first post of a topic is the topic.
When a first post of a topic is spam, I usually delete using the topic wrench as opposed to the post wrench. I have noticed that other moderators at times delete by using the post wrench on the first post, but have not noticed any ill effects. It sounds like you may have uncovered a problem. (possibly a can of worms)
Can you list the exact steps needed to reproduce this?