There are several points of contention but here are some of the most common points I've encountered:
- When the term 'sandwich' is used in reference to a bagel being used for the 'bread' there is a presumption that the bagel is sliced along a transverse equatorial plane with the hole of the bagel being orthogonal to the ground. This photo clearly defies that convention.
- A sandwich by most conventions is kept between two or more slices of bread in layers. If there are more than two layers between bread in a sandwich they are stacked vertically and separated by individual slices of bread. The subject under discussion has two layers, which is conventionally acceptable for a sandwich to have, but they are not stacked vertically. Some scholars have argued that this means that it is incorrect to call this a 'sandwich' in the singular sense but it is acceptable to call the entity a composition of 'sandwiches'.
This is just a taste of the great bagel debate currently raging across very small niches of geeks but hopefully it gives you a flavour of why no definitive answer has been accepted by the community. And this has not even touched on the bagel-complete problems such as the topological properties of the mobius bagel http://images.tastespotting.com/uploads/thumbnail/125155.jpg.