That is a book definition. That's why you keep repeating that 'otc does not handle short', which is true, but not applicable in day to day trading perspective.
When you buy a stock, the trading world presumes, by default that you believe in the company and its long term prospects (1 year at-least) and that the price of this stock is going to go higher.
At some point, you start to 'feel' (by whatever means : that of financial statements, news, volume or sentiment) that the value of the stock is going to go low for whatever reason (maybe it has gone high too fast, or you feel the company is not doing it right) you decide to sell it and hence you are (short)selling it. The trading world then redefines the stock as a short-sell. And hence this is how your broker's statement change.
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