SegaNet was Sega's attempt at offering an ISP for gamers to play online. It's not really comparable to Gold/PS+. It was primarily an ISP, which is why the cost was so much higher. However, after SegaNet ceased to exist, Sega did radically change their online policies and the games then became pay to play online. That fee, IIRC, was $10 a month.
Yes, you could have bought a game with online play free out of the box, and then months later a monthly fee was required to play the game. But even shadier than that, when SegaNet went under, Sega sold all those customers accounts and contracts to Earthlink, the shittiest ISP that was around back then.
Some people complained, but for the most part, no one cared. Just like with this whole XBL PC thing. It's just something some people on the Internet will bitch about, but most people understand. Remember, if the forums on the Internets were a representation of how the average person thinks, CoD would sell like 59 copies a year.