LEDs produce directional light (unlike a filament lamp, where the light is radiated all around and then focussed by a reflector). Each lamp will have a cluster of LEDs and, depending upon their arrangement, the light from them can be focussed into a very narrow beam or a very broad one. A too narrow beam might mean that the lights are only visible from almost directly head on (rather than from at an angle) and could possibly even be dazzling to someone whose eyes fall within that beam. A too wide beam could mean that, through light being widely dissipated, the visible output at any particular point in front of the lights isn't bright enough to meet regulatory requirements.
Therefore only LED lamps which have the right type of beam arrangement (and have therefore been 'E-marked) are permissible.