What you describe has been my experience after my original brokerage firm, which was Quick & Reilly, was absorbed into a different company. I traded a lot, with absurdly great success, in 1999-2000 during the internet bubble, and I was deeply on margin throughout that period. Q&R reduced my margin-subject-to-interest by the amount I held in short positions on any particular day. I'm not saying it's been like that since then (or with other brokerages back then, since I don't know). In fact, for me it hasn't been: since the disappearance of Q&R, any interest on money received from my opening a short position has accrued, like you say, to the brokerage. Ah, the good old days.
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