Why is so expensive to buy Bitcoin with Credit Card? There are many sites accepting credit cards to buy Bitcoin, but they charges a huge fee, sometimes is more than 10% (and I think never less than 5%).
Why is that? what is the problem with buying Bitcoin with CC? because usually buying via bank transfer has no extra cost.   

I think could be related to some chargeback protection, but if this is the case, is not possible for example to block the "withdraw" of BTC from the seller account for a few days (or whatever is safe to avoid chargebacks)?

Sorry if it's duplicate but I couldn't find the answer neiter here nor google :(
It is about the blockchain. A transaction goes in to a block and gets stacked on the blockchain. This process takes a lot of cryptographic processing power, which is done by miners. This process becomes practically irreversible as more block go up the chain.

So the power of bitcoin is its guarantee a transaction is becoming irreversible and definitive as the blockchain grows. This means, you buy Bitcoin it is yours. Yes, if it is still in the exchange they hold the private key to respend your BTC, but they won't hold the lock forever.

Chargeback is one thing. A stolen credit card another. Credit card fraud is at large scales currently. All this is putting a big administrative overhead on the exchanges. Users will have to do all sorts off verification to prove the credit card is really theirs. So, that 10% is the administration fee for all that, plus risk mitigation. Also I believe, that withdrawal is restricted anyway, regardless of this fee.