I would post this as a comment but not enough rep, so:
You can think of the sampling requirement arising due to the highest frequency of a particular signal. When you use the term "bandwidth", you're looking at the width of a signal's spectrum, which is the twice the signal's bandwidth, i.e. the highest frequency minus the lowest frequency.
In the example of the sinusoid, the "width" of the spectrum is still the difference of the maximum frequency minus the lower frequency which is twice the signal's bandwidth, just like the previous example. It just so happens that the highest and lowest frequencies are the delta functions at $\delta(f + f_0)$ and $\delta(f - f_0)$ and the sinusoid is centered at 0 frequency. So the bandwidth is simply the highest frequency delta in the spectrum, located at $f_0$. So in this case, "bandwidth" and "highest frequency" are referring to the same thing.