During a session a player asked if it were allowable for the bullywugs to have 3 unmodified attacks, claiming that even monsters natural attacks should be imposed with a -2 and then a -4 to hit penalty for each subsequent roll. I stated that since 1st edition the monster manual states that "NUMBER OF ATTACKS shows the number of basic attacks the monster is able to make during a given melee round." and that he was confusing the penalties imposed for multiple attacks are for player characters that learn that skill, where monsters multiple attacks are natural and not learned, thus not needing any to hit penalties. Is this still valid or did Second Edition add something that I missed?
Monsters suffer no penalties for using all the attacks their entry grants them. You're correct that the two-weapon fighting penalties are limited to humanoids that have to learn how to fight with two weapons.
This is part of the general trend in TSR's D&Ds, from the original game up through AD&D 2nd edition, for monsters and PCs to operate by similar but different rules that are tailored to what the game needs from those different roles. In the case of monsters, everything that normally applies to them is already accounted for in their monster entry, with any exceptions explicitly called out.