You are asking an unrelated question to the post. By the way blocktrades is a business that is not directly related to Steem (although the owner of blocktrades.us runs a witness node on the steem blockchain they also partner with other platforms such as bitshares.org).
I guess that they charge the fee to cover the cost of the Bitcoin network and to make a little profit (have you seen the fees to make bitcoin transactions..they are insane).