Are molcajetes brought to the table, or do these people sneak into the kitchen to steal stuff?
Stealing a Molcajete and Tejolote (that don't normally come out to the table) are not easily tucked into a purse (unless it's one of those big enough to hold a Baby or a Poodle) or a bag of leftovers. Well, maybe if it's one of those little itty-bitty ones that holds 2 Tablespoons of Salza or Pico. If you want one then just buy one they're pretty cheap. But then you have to spend months working on breaking it in.
To answer Your question, some places will bring one out to the table but I've been to hundreds of Mexican Restaurants and maybe 10 or so bring them out to the table.