I think it comes down to the fact that most people these days do not go out of their way to buy a TV with 3D. If it has the option, then that is a bonus.
Because of this, it is better for them to sell without glasses, as they can then reduce the TV price by £50-100 making it more inviting.
Also, if you are selling a big TV for a family living room, how many people are going to be watching it at any one time? If they give away 1 pair of glasses for a family of 4, it will annoy people more than not getting any.