Why Does it Seem Like I Am Always buying on Margin?

The short version is that IB ( Interactive Brokers ) always calculates your position as the 'worst case' as in using the most margin. They calculate it this way but you probably are not paying interest.

From IB is the long version

IB calculates and displays a margin requirement on the entire portfolio of positions held in an account even if the account holder has paid for the positions in full and has not borrowed any funds to effect their purchase. This is done to support real-time credit check processes and is necessary to determine the Available Funds (defined as Equity with Loan Value less the Initial Margin Requirement) on hand to support any subsequent trade activity, position transfers or cash withdrawal requests.

By the way, the display of a margin requirement does not necessarily imply that you are being charged interest. Generally, a negative cash balance is indicative that one is borrowing funds to support securities purchases, however, one may be subject to interest charges even if they maintain an overall net long cash balance (as proceeds from short sales require special consideration and one may hold a long balance denominated in one currency and a lesser short balance in another. See the following link: http://ibkb.interactivebrokers.com/node/667)