The reason that preemption is disabled on a uniprocessor system is:
If not:
P1 holds the lock and after a time is scheduled out.
Now P2 starts executing and let’s say requests the same spinlock. Since P1 has not released the spinlock, P2 must spin and do nothing. So the execution time of P2 is wasted. It makes more sense to let P1 release the lock and then allow it to be preempted.
Also since there is no other processor, simply by disallowing preemption we know that there will never be another process that runs in parallel on another processor and accesses the same critical section.