Why don't people bother to read the small print before they sign the contracts?
ALL credit cards charge interest from day one on cash withdrawals and those ghastly credit card cheques.
The reason is quite simple. The credit card company earns at least 2% every time you use your credit card to buy something. The store that accepts your credit card payment pay it.
Barclays, and all other credit card companies, are providing a service for profit.
If they did not charge you for cash advances, stores would simply ask you to withdraw money from the nearest cashpoint to avoid these charges, perhaps offering a small discount as an incentive.
The interest rate is always very high on cash advances, at least 3% more than the standard interest rate. This applies to credit card cheques too - which in my opinion should be shredded upon receipt and never used.