Quote from Risk3:
If fx "brokers" would only facilitate clients' orders (for which they get a fee - the bid and ask spread) and not be in the business of taking their clients' money, then the spread itself, though expensive, would not be an issue.
The problem is that these fx "brokers" are market makers, they trade against their clients, making profits as a direct result of their clients' losses. FX market makers execute client orders against advertised prices on a trading platform they own and may not always take the executed orders to the Interbank market (retail size is way too small for this market) but instead act like bucketshops and pool them internally or just take the other side of the trade if the odds are right. And let me tell you, the odds are heavily tilted towards the market makers as they can easily manipulate the rates received from the banks, shift bid-ask spread in the direction the currency pair is moving, put in rates that do not reflect market price thereby triggering (stop-loss) orders etc. After all, they own the trading platform, know their clients' position, and can do whatever they want.