Why regular user with realtime priority can't run pthread_setschedparam(3) with sched_priority=10 ?

[Yuri]

The process (jackd) calls this code:
 >        rtparam.sched_priority = priority; // =10
 >        if ((x = pthread_setschedparam (thread, SCHED_FIFO, &rtparam)) 
!= 0) {

It succeeds when the process is run as 'root'.
It fails when the process is run as a regular user with realtime 
priority (set with rtprio 0): Operation not permitted

Why realtime priority of the process doesn't make high priority threads 
possible?

Yuri