Bunds are margined as if they could go to -14% YTM overnight. I guess IB doesn't want people competing with 1.5% margin interest by shorting bunds.
Trade the future? What's the face value in dollars of the bund? They're probably forcing you to buy it outright. I bet the future is an order of magnitude or two cheaper. Instead of complaining you could just call them and ask why the margin is so high. IB makes so much money I doubt they care if your teeny tiny account shorts a bund. You're no goldman.
This is why I've stopped trading IB. I still occasionally put on small trades. I can't swing the size I like to anymore.
That's why it is important to have multiple trading accounts. I haven't use IB for months. by now, I'd have forgotten the password. go try EUREX - euro buxl and you will be surprised at its margin requirement.