Some motorcycles are restricted to 300km/h, but not all. In 2000, the Japanese manufacturers had a gentleman's agreement to limit motorcycles to 300km/h. The Suzuki Hayabusa was already on sale, capable of more and the big four manufacturers were nervous that if they didn't impose a speed limit on themselves, someone else might introduce a far stricter one.
From 2001, Suzuki Hayabusa's were restricted, and Kawasaki's ZX-12R was the next to be limited.
Since then, several Suzuki and Kawasaki motorcycles have been made with top-speed limiters: both Hayabusa models, the GSX-R1000, the ZX-10R, the ZX12R and the ZZ-R1400. Limiters are just on top speed and don't affect power or acceleration in lower gears.
Honda says its motorcycles aren't limited, and the latest 2012 Honda Fireblade can exceed 300km/h, at least in theory. Yamaha says its motorcycles won't hit 300km/h but that's because they're designed for decent mid-range, not purely for top-end. The Europeans are less conservative. MV Agusta is so proud that one of its motorcycles can hit a claimed 312km/h they've named it the MV Agusta F4 1000R 312. There's no top sepeed restrictions on any European motorcycles, as far as we're aware – but we wonder if BMW may introduce some, seeing as many of their cars have a voluntary 250km/h restriction.
The last gentleman's agreement the Japanese manufacturers entered was the 125 horsepower limit in the early '90s, which they abandoned after a few years. So how long will the 300km/h agreement last.