It's the practice that's different. Depending on the country you're from, you pay a certain yearly fee in order to be accredited. If you pay that fee, your company can request the national body to accredit a certain person so that they can attend a particular ISO meeting. It's different in the details. Standard bodies are not charging money.
Standards bodies have to get some money in order to do the job they have to do.
SDMI is no different. We received some feedback from a management company that they tried to join and were rebuked. You must have a concrete business interest in secure digital music. People managing rights for artists are indeed legitimate parties. They don't have a specific business interest in secure digital music, but they are legitimate.