Cause: That stinkin' top bolt!
How to fix: Take a 3/8" drive 17mm shallow socket and streamline it. Shorten up the hex side of it to just the depth of the bolt head (~.25") and then grind off some of the shoulder on the ratchet side. Use a small diameter extension that's at least as long as the starter (if it's too fat, you'll have some clearance problems).
Your tool should look something like this:
When you're using this tool to take a starter off, you'll want to take the bottom bolt out first because when the top one starts coming out it will trap your tool between the bolt and the starter body if you can't pull the starter out of the block.
Installation: I suggest using a bit of Anti-Seize goop on the threads of the bolts you put on so it'll be easier if you have to take the starter off in the future.
-Michael