Some types of lilies just naturally grow very tall - trumpets and aurelian hybrids are examples that can easily reach 6'. The stalk with as much foliage intact as possible needs to stay in place until the foliage dries and turns brown. The bulb needs it to photosynthesize and create sufficient energy to produce next season's flowers. The recommendation is not to remove more than 1/3 of the stalk when cutting for indoors or removing spent flowers. Cutting back further will not make next year's plants any shorter but it could very well prevent them flowering properly.