Because it's S1 server. People with S1 license can drive in S2 (physics wise) server. The old LFS S1 0.3 had 15 slots maximum. 15th slot is reserved for an admin, I think.
When the next LFS patch arrives I will be increasing the STCC S1 server to 18 race slots and 22 connections, also the auto-kick for drivers who spectate for too long will be sorted out, i'm not 100% sure if it works.