Grossly simplifying it ...and in general ... Inop is allowed because the thrustreversers are technically an 'extra'.
Inop is not the issue, but one single T/R inop. Russia expliciltly banned it after the S7 Accident in Irkutsk, it´s either both or none.
Not because it is a general issue as a single event, but it can bite you if something else goes wrong.