Because "code it if you know how or pay someone to code it" is not an answer when talking about software and lack of features, even if it's OSS.

OSS software is built by its community of developers, not by its community of users (even if those overlap a little, the second is many orders of magnitude bigger). So when someone criticizes it that it lacks feature X, telling them to "code it" themselves is not really helpful. They already know they have this option. If they had the knowledge, inclination, interest and time, they would have done it already. Not to mention that even if they tried, community politics and roadmaps might be against their wanted feature.

Can we accept that someone is free to criticize a piece of OSS software without being obliged to fix it himself?

But these kind of critics about known problems don't add any value.