The lenses which make up the scope will always reflect some light (~0.5%).
All optical materials will reflect some amount of light. This is due to the fact that light travels slower inside of the medium than it does in the surrounding air. The slowing down of a wave always results in some reflection; in optics it is governed by the Fresnel equations, but there are analogous reflections in electronic circuits and waves traveling along a string.
There are techniques which use destructive interference between multiple reflections at successive boundaries to give optical devices very, very low reflectivities at a specific wavelength. However, for a rifle scope it is important that the reflectivity be low for the entire visible range. Browsing the coatings offered by Edmunds shows that a good coating can reduce the reflectivity down to about 0.5% across the entire visible range.
The lens tube cannot simply be extended without degrading the image quality.
There is a lot of information on hoods on photography.SE (here and here for instance), but I'll briefly summarize. The length of the hood is limited by the field of view. Essentially, light from every point in the scene needs to be able to reach every part of the front lens. 538
I.E. it is not enough for the light to reach only the center portion of the lens.
For this reason, simply extending the tube beyond the front of the objective lens is not an option because light from the edge of the scene would not reach the edge of the lens. The hood must have a radius which is larger than the objective lens itself before it can be extended; otherwise you are throwing away light from part of the scene. Pay attention next time you see a professional photographer (or enthused amateur) with a lens hood and you will immediately notice this. The most effective hood is actually a cone with an angle set by the field of view which will start to make the scope very large and bulky.
The reflection is less noticeable than you might imagine.
Since the objective lens of the scope is a focusing lens, the outer surface is curved like a diverging mirror. This means that the light redirected toward the enemy sniper is significantly less intense than it would be for a flat mirror. For a flat mirror the entire surface of the mirror redirects light from the sun to the enemy's eye, while a curved mirror has a much smaller area which gives the correct angle between the enemy's eye and the sun. 538