Doesn't sound right....
The preformed base shouldn't have a liner and shouldn't need one - it is fully waterproof on its own. If the floor isn't flat and level, then setting the pan in mortar is an easy way to make it nice and level and fully supported.
What brand and model pan is it? Do you have it? Did you read the installation instructions to see what the manufacturer says? If not, can you find and read it online?
A preslope and liner is used when you are building a traditional shower...a preformed pan isn't a traditional shower. The reason the liner is needed is that the tile is not considered fully waterproof, and the preslope with the liner over it directs any moisture that does get in down to the weep holes in the drain. The drain used on a preformed pan has no weep holes, so if any moisture did get in the pan, it would just sit there and fester since it has nowhere to go.