Why does the AAMVA CDL Manual advise charter bus drivers not to let riders board the bus before the posted departure time?

49 CFR 383.117(a)(6) (operating practices not otherwise specified); AAMVA CDL Manual Section 4.2.4 "At Your Destination," p. 4-2

An unattended charter bus full of unsupervised early boarders is an opportunity for theft of rider belongings or damage to the vehicle before the trip even starts. Knowing the manual's stated reasoning, not just the rule itself, helps a driver explain and enforce the policy confidently with impatient passengers.