Well, the oscilloscope is capable to sample every 10ns, but it won't be able to detect a signal with a full swing in less than 17ns. Effectively, you won't be able to measure rise times smaller than 17ns, even though the sampling period is 10ns. This limitation is due to step response of the analog input circuit of the oscilloscope.
The 10ns sampling rate is a limitation of the digital input circuit (and other things).
The Nyquist frequency limit is used for signal reconstruction, but if you would try to display a signal sampled at a rate smaller than 2*f, you would deal with aliasing issues. If you don't have at least a few samples per period, you won't be able to make any reliable measurements.
The oscilloscope bandwidth is 25Mhz instead of 50Mhz(Nyquist frequency) because of the analog input circuit of the oscilloscope. Signals with frequencies higher than 25Mhz will be attenuated more than 70% (-3dB).