Yes, it is sad.
Canon 284: Clerics are to wear suitable ecclesiastical garb according to the norms issued by the conference of bishops and according to legitimate local customs.
Deacons and priests are clerics, and therefore have the right and obligation to wear the cassock, clerical suit, or whatever has been determined as clerical garb in that locale.
Canon 288 releases deacons from the obligation, unless it is asserted in force by local law, but they retain the right to clerical garb. There is nothing that indicates a bishop has the right to deny clerics their right to clerical dress.
Seminarians, like all lay people who are not clerics, are forbidden from impersonating clerics, ie, wearing clerical dress. However, this rule seems to be ignored frequently. Personally, i think it should be more strictly enforced. A seminarian or deacon candidate is still a student. You don’t get med students calling themselves doctor or law students impersonating lawyers. Seminarians are not priests, and are still lay men until they are ordained.
Alternatively, other lay people engaged in full time pastoral ministry should be allowed clerics, or some adapted form of them. If a student can wear clerics, why not the person already engaged in the kind of ministry the student is preparing for? (E.g., a pastoral associate, or hospital chaplain).
One difference common in the Byzantine tradition is that celibate clergy (deacons or priests) wear black, whereas married clergy (deacons or priests) wear grey. This seems like a good way to respect both the clerical state, and the state of life.