In our observations as Deaf ASL instructors, hearing students typically entered and went straight to their seats in ASL classroom without explaining anything.
Whereas, in Deaf space (e.g. Deaf teacher and Deaf students in classes), a Deaf student who arrives late, usually briefly explains why she/he is late. Bad traffic. Sometimes, they explain nothing for some reason, "Sorry, late". There are exceptions such as conferences, large workshops, etc.
One possible reason for this cultural norm is that everyone knows one another in the close-tight community.
"The part I loved the most was learning that when someone comes into a classroom if he or she is late, it is explained why, as opposed to just rushing in and sitting down. That is so polite and genuine." -- ASL 112 student, 2014.
Politeness is culturally relative and different in cultures. What the very same action may be polite in one culture but may be impolite or inappropriate in another culture.