because it mean also less trade
Well, how will you convince your customers to trade more if you increase your trading fees and charges?
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0.01% per 4 hours = 0.06% per day
0.06 x 365 = 21.9% per annum
Greed!
Charge 21.9% per annum BTC/EUR longs
+
Charge 21.9% per annum BTC/EUR shorts
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Makes 43.8% interest rate differential between longs and shorts
Greed^2!
Is Kraken the greediest exchange among all bitcoin exchanges?
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Greed can explain many things but not everything!
Are BTC and EUR financing both equally available? Not at all. EUR funds are much easier to find at nearly zero interest rate while BTC funding is more difficult to secure, especially long term, resulting in considerably higher interest rate. Applying same rollover fees for both longs and shorts means you stimulate BTC/EUR shorts and punish BTC/EUR longs.
Why would a bitcoin exchange side with bitcoin enemies?