All great theory...Go short your positive terminal to the frame and tell me what happens WITH the key on. I bet you don't think a branch circuit can trip a main breaker either.

In these DC circuits the overcurrent protection has to come after the starting circuit, otherwise you would blow the fuse or have to have a fuse too large to protect the system. This however does not mean that a short anywhere in the system will not blow the main fuse, it is still a short on the system and will find the weakest link, that will be the fuse if the short is not removed.

For your theory to hold water there would have to be zero continuity between the wire from the battery to the starter and the wire from the starter to the main fuse. So unless you think that connection is dropped once it is started, you have a very real possibility of a short on that part of the circuit blowing the main fuse.

I am done with you.