The Streetside View button is not always vissible
Only when Street View (or “Streetside View”, as the Microsoft Bing Maps Team calls their Service) is available for your current position (that is: the center of the map) there will be the Street View button:
To be honest, Street View does not exist for every location on planet earth. So when there is no Street View available, the button will not be available:
We could have left the button untouched and could display some kind of error message, but we think hiding the button is way more elegant. However, we acknowledge that this could be a little bit confusing as long as you don’t understand the reason behind this.