Maybe they only allow a certain number of simultaneous connections to their server from one connection. To try and prevent leeching of their content and bandwidth.

You mention Halo and Gears of War as two examples of modern gaming, but I personally don't think either of those hold up as good examples. While GoW set new graphical standards on release to show what the Xbox 360 could do, and the original Halo pushed the first person shooting genre forward, showing that much more was possible compared to similar games before it, they are both at their hearts just shoot 'em ups, and if you get right down to the nuts and bolts, they owe everything to games like space invaders and galaga. Take away the graphics for both and you are left with little more than shooting moving targets.

In contrast games on all retro platforms had to do more than rely on shiny graphics immerse the player and get them hooked. The Spectrum is one of the greatest examples of this as it was the most underpowered and graphically inferior 8-bit system on the market, but managed to see some amazing games that are still as playable today as they were on release. I don't think the same will be try when looking back at Halo or GoW in 20 years time.