SEPTA did charter buses years ago. When I was in college, my lacrosse team would sometimes go to away games on a SEPTA charter. But some time ago, maybe about 1990, maybe earlier, UMTA placed heavy restrictions on use of federally-funded equipment in charter service, specifically to prevent unfair competition with private operators who did not get federal subsidies.
Charter operations were not completely banned, but before a public agency could accept a charter contract, it had to publish notice of its intent to do so, and determine there were no private-sector operators able and willing to provide the service.
There are still occasional instances where the public agency does meet the requirements and charters buses. One recent example, which I happen to recall because there was some controversy over whether the regulations were followed, was the Rochester NY RTA providing parking lot shuttles for spectators at an LPGA golf tournament.
SEPTA does still provide railcars (subway, trolley, and RRD) for charter, and tariffs for such service are published. Don't recall if they're online though.
This regulation may also have been the impetus for SEPTA formally converting its special school routes into nominally-public 400-series routes, and for publishing formal tariffs for Mann Music Center service.