Sage’s decree in linen tzitzit

Greetings to the Rabbi,
A principle that appears in many books and authors is that the sages have no power to permit what the Torah explicitly states is permissible (even through a sermon such as – and on the eighth day – on Shabbat – see 16 Tikvah).
So how did the prisoners forbid wearing a tzitzit? After all, this is a clear teaching for permission? (In Yerushalmi it is written that a tzitzit shall not be worn, and in one text it was stated…)