So you have a domain name which has an A record pointing to a tumblr log at 126.96.36.199?
I just wonder then if it is something that a tumblr theme is doing.
I looked at the tumblr log site and see that you can add a custom html theme via the dashboard but you need to use special variables and blocks.
I would try a quick message to tumblr support to see if it is their system that is changing your meta tags.
I have put your robots meta name tag into a DreamHost HTML page and it has not changed yet.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK! :@
Act on my advice at your own risk!