They do not need salt licks because there is salt in the pellets they eat. Too much salt can cause extreme thirst. Too much salt can increase blood pressure and cardiovascular problems and in some cases too much salt can lead to death.
Cavies are lactose intolerant meaning they cannot digest dairy products very well. Yogurt drops contain dairy products.
It's not salt licks that cause stones, it's mineral licks. The main mineral in mineral licks is calcium which in turn can and does cause sludge/stones.