debashish,
this is a radio parameter that is transmitted in the BCCH. and changing this timer can have a big impact on the radio load. for this reason, the timer is set at BSS level, not NSS.
with all due respect : why do you question 3GPP implementation ?
the point of GSM engineering is to go along with the 3GPP rules, otherwise you’ll spend too much time complaining and little time actually optimizing :))