How can an employer decide they're not going to cover particular drugs any more? It's easy. All they have to do is contact the insurer and tell them. No law says that they have to go on carrying the same prescription benefit forever. Whether they have to notify the employees and if so, under what time frame is a matter of state law. I am also of the opinion that there is a great likelihood that you WERE notified and simply did not read the notice. I base that on many years of experience - as I said in a post a couple of weeks ago, if I had a dollar for every time an employee claimed I did not notify them of something and I pulled out a copy of the notice, sometimes with the employee's signature on it, I could retire two years early.
As for how you could still be paying the same rates, that's easy too. Prescription coverage is EXTREMELY expensive and going up all the time. (After all, they have to pay for all those commercials where they tell you to ask your doctor for Drug X without telling what Drug X is for, somehow!) It is most likely the dropping of these drugs that has allowed them to keep your premium rates the same. In other words, if they had not dropped these drugs, the rates would have gone up.
Again, I am basing this on many, many years of administrating employer health insurance benefits and also on five years of experience working for a national health insurance carrier.
One other possibility. It is ENTIRELY possible (in fact, this is the most likely possibility of all although Aetna would not tell you this) that the change was initiated by Aetna. They would have told your employer "You can have this Rx benefit for this price, or that Rx benefit for that price". There would have been a SIGNIFICANT price difference between the two, I promise you! Your employer, not unreasonably, choose the lesser and less expensive benefit. On that basis, Aetna would tell you that your employer chose to drop the drugs, whereas actually your employer chose one of two Rx plans. I am not stretching things; I have seen this happen.