The correct response is:
it is considered rude to watch two people who are having a conversation in ASL, while it is somewhat more appropriate to glance at two people who are having a conversation in spoken English.
This illustrates a cultural difference in communication styles between the Deaf community (using ASL) and the hearing community (using spoken English). In ASL, the conversation is usually intended for the participants, and onlookers are often seen as invading that space, whereas in spoken English conversations, it's more common to engage by glancing at others.