There are rules against crew members partying too much, especially with guests, but it's not uncommon to see them unwinding and letting go just a bit. A large part of this has to do with just how much they work and how little they actually have to spend on drinks. They usually pay around $1 to $2 per drink. Compare that with passenger drinks which can set you back $10 or usually more, and it's not hard to see why crew members might be the life of the party.
Still, most cruise lines will have at least a couple of clauses in their contracts that dictate how much crew members can drink and how much they can interact with guests.