There was a court case about this actually, lol. A cyclist was caught by a police car going above the posted speed limit(I think it was 30km/h where he was), and he refused to accept the ticket. The case went to court and he argued that since the law doesn't require bicycles to be fitted with any instrument to measure speed, he is not required to know the exact speed he's travelling at. And weirdly....the court agreed and the ticket was cancelled.
Edit: so, yes, no speed limit for bikes, but speed limit for ebikes.
You can actually buy e-bikes going up to 45 km/h but they are regulated as scooters and has to be registered (usually free), requires a drivers license and the use of moped helmets and you're usually not allowed to use them in bike lanes. Oh, and you need insurance too. So for most people it's not worth the hassle.
The reason you probably see a lot of 25km/h ebikes is that in the EU, anything faster than that is regulated as a moped and requires a license, so I imagine the 25km/h bikes sell more.
The reason why even the faster bikes are 45km/h is also probably because that that's the maximum speed of a bicycle in the EU.
[0]: https://gazette.govt.nz/notice/id/2013-au4618
[1]: https://www.nzta.govt.nz/vehicles/vehicle-types/low-powered-...
[2]: https://www.giant-bicycles.com/nz/fastroad-eplus-ex-pro-45km...