The difference in the questions is buying vs. short selling. Here are some similar examples to illustrate the difference:
Buying 100 ABC @ $15 in a margin account
If the stock value sinks greatly, the maximum amount of risk is the purchase amount of $1,500 (100 x $15). Since the risk is limited, the $2,000 minimum doesn’t apply.
Short selling 100 ABC @ $15 in a margin account
If the stock value rises greatly, the maximum amount of risk is unlimited, so the minimum of $2,000 in account equity does apply.
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