My HDB block TOP on 1998 and unlike newer HDB, there is no utility box inside the unit. By any chance is your block TOP on the similar year?Not sure about your case. For my house the main splitter is definitely inside the locked box as I got a good look years ago when I subscribed and then unsubscribe to starhub TV.
The reason why they locked the box is that there is 2 main splitter inside. One from the rooftop master antenna and the other is the splitter from Starhub TV. If you subscribe to starhub TV, they will just plug your house main splitter to their starhub TV splitter and you will be able to watch their programmes, losing access to free to air TV signals from the master antenna in the process.
I think you should call town council or HDB to verify whether your main splitter is inside that box or not.
Before i reach out to starhub, I have actually called HDB then bounced to TC then lastly to starhub.