zen fire is just a package name only like a brand. therefore, people with zen fire are paying the brand name. whoever came up the name zen fire is making lots of money by charging the commission higher.
edit: Imho, mirus, amp, and dorman offer zen fire, while the rest offer rithmic. if u are looking for cheaper commission, get the ninja rithmic. that will cut the middleman
there are lots of brokerages offer ninja rithmic. click the link below:
http://www.rithmic.com/partners.html
there are tons of brokers offer rithmic or ninja rithmic. you notice that, the name zen fire below the technology group.