You may want to broaden your understanding of friction. It may be dynamic or static (and in the case of glaciers, both certainly exist). Both produce heat, as does torsion within the ice. Fissures can be a melt water feature - while the fissure may have started as a surface crack, running water and frost heave can certainly erode ice surfaces.
That there is a degree of friction exists beneath glaciers is a fact. But what also is a fact is that the primary source of meltwater from glaciers is from the surface melting. Another fact meltwater sources from the base of the glacier are heat generated from ice deformation - which is a form of pressure increase within the ice pushing it past the ice-liquid pressure/temperature boundary as well as other pressure factors. Now if you want to call that 'friction' I would agree only in that it is causing pressure changes in the ice itself. It is not kinetic friction but approaches the realm of fluid dynamics because of the plasticity of the ice under pressure. It passes the use of the term 'friction' in the common use then.