You're right. It's not very different and I think this is perfectly legal if you register as an ATS and comply with Reg ATS. Reg ATS is not even all that onerous if you are already a broker dealer, but at a minimum it includes reporting trades correctly and telling your customers you may take the other side of their trades.
It is a "technical" crime. There are good reasons to regulate ATSs, and the brokers didn't follow those regulations. Instead of prosecuting the brokers here for an obscure crime like "operating an unregistered ATS", the SEC decided to prosecute them for fraud. Judges know what fraud is and as Matt Levine puts it in other articles, "every financial crime is also securities fraud".
It is a "technical" crime. There are good reasons to regulate ATSs, and the brokers didn't follow those regulations. Instead of prosecuting the brokers here for an obscure crime like "operating an unregistered ATS", the SEC decided to prosecute them for fraud. Judges know what fraud is and as Matt Levine puts it in other articles, "every financial crime is also securities fraud".