When the experiment began, there was really no treatment. Partway through the experiment, penicillin was discovered. Treatment with penicillin would have cured the men, but it was withheld and the experiment was continued for 25 additional years, with no treatment.
Not ethical. At all.
There is still dispute as to if and why penicillin treatment was withheld. Some doctors, admittedly with an interest in the case, said it was thought to be too late to have effect. As I said, I have no way of telling who is right or wrong in this, but many of the politicians who have jumped on the story (Clinton, Kennedy)are known liars who would never miss a chance to exploit a tragedy.