I just want to add that I get the same problem with any of the other Dreamweaver Extensions on CS4... Any other extension seems to uninstall fine.
ALSO: I just tested CS6 and tried to remove MySQLi from there. I get exactly the same results, except the error I get is:
At line 140 of file "C:\Program Files (x86)\Adobe\Adobe Dreamweaver CS6\Configuration\Commands\WA_unInstall2.htm":
shareMatch has no properties.
I must say - I do not like your newer installation methods. I really wish we could just download and install completely offline like we used to be able to do. I'm not sure if it's related or not with this problem, but I certainly don't like being so dependent on your server being up and healthy.