Re: Darts -- If they weren't finesse, they would be strictly inferior to daggers in my humble opinion: same range, same damage die, same damage type ("piercing") -- but daggers can also be used in melee while darts cannot. Darts have to have some usefulness, or else they become a mere legacy weapon that nobody would ever actually use. Given that comparison, making them be "finesse" costs nothing, and might help people choose them in some cases.
In looking back at earlier editions:
3.5E does not list weapon restrictions on the Sneak Attack ability, but does limit Rogues to a fixed list of available weapons: all simple weapons, plus the hand crossbow, sap, shortbow, and short sword. This is somewhat comparable to the 5E list for the Rogue, although the sap, short sword, and shortbow are Martial Weapons in 3.5E, and the hand crossbow is an Exotic Weapon.
4E limits the Sneak Attack ability specifically to "a light blade, a hand crossbow, a shortbow, or a sling," although the "Ruthless Ruffian" tactic allows the use of a club or mace as well. 4E didn't allow SA using a dart, but also didn't include "Dart" as an available weapon at all.
From a roleplaying perspective: Guran initially loaded himself down with three types of thrown weapons: a dagger, two handaxes, and a light hammer. He could do Sneak Attack at 1st level, but he may not have enough evidence available yet to be able to conclude that sneak-attacking works with the dagger but not with the other two types of thrown weapons.
If he lives long enough, he might find used daggers in some of the found treasure (if the party finds any treasure), and slowly add them to his arsenal. (He would always keep at least one handaxe and one light hammer in order to have slicing and bludgeoning capability.)