Hmm, that's interesting. All the other monasteries I have visited are open for quiet respectful public people to attend at any time.
Ah, except for one Tibetan monastery (Gelug), where I could not attend the chanting because you had to have been initiated, or have had a transmission or something. Maybe this monastery is the same, since it's a bit Mahayana. You could ask over on the more Mahayana http://www.dharmawheel.com
Sorry, I don't know about any suttas, but it would seem reasonable for the monks and residents to have a nice private time once a day, especially if they are in a busy monastery popular with the public.
I know of some Ajahns who give private talks to their monks, and if a member of the public wants to attend, they have to formally apply to listen.