However, it seemed to be one guy that wanted to do that, and the trial was held to examine if that would be right or not, and to establish the legal precedent.

At the same time, Starfleet also enabled it. The entire case would have never happened if it had just been Maddox asking for Data’s voluntary participation. Part of it was that Starfleet was trying to compel Data to submit to the procedure, and also prevent him from leaving Starfleet to avoid it (hence the property angle).

We also know that the ruling was constrained to that one case both from Voyager, where it was outright stated to not apply to the Doctor, and because Data also had to fight Starfleet to prevent them from taking away Lal. While the fight was ended early as Lal died (possibly as a result of the emotional stress), it would not be too surprising if another legal battle resulted. Maddox might have started the events of Measure of a Man, but he was not singularly responsible for that whole business.

Wheras with the Pegasus, the investigation disappears into a hole and not touched since. Instead of punishing Pressman, he gets made Admiral.

We don’t actually know what happened from after the Pegasus’ cloaking device was revealed, other than that Pressman and the rest of his crew were likely to face court martial (and Pressman had “high-up friends” in Starfleet). He was promoted to Admiral before it was exposed, and it’s unclear what he was promoted for, since he was already Admiral when he tried to get it back.