To  put it another way, imagine that your neighbor prepares taxes for a living and you are an auto mechanic.  You know that your neighbor charges his clients when he prepares their taxes.  One day he asks you if you'd be willing to fix his car in exchange for the preparation of your taxes.  If you decide that the exchange of those services is a good trade why would it matter that typically he charges money for his services?

The key word is exchange.  Whether you pay someone for a lesson or have a free language exchange, in both cases something is given for what is received.  With a paid teacher/tutor there is no exchange of language information.  All of the teaching is in one direction, toward the student.  In exchange, the student pays the teacher/tutor for their time.  In a free language exchange both parties benefit from the time spent together so no money is required.

I don't know if I get you, it's a bit confusing to me! 

For instance, I am a community tutor here and I am learning many languages, if someone who came to me saying " Hey Murillo, I see you are learning French and I want to learn Portuguese, let's have a " SERIOUS " language exchange? ". I would, no doubt! 

But I would not offer free language exchange to someone who charges here because I don't know if the person is really interested, some of them say that they're learning a language, but in reality, they're not serious about it! 

Amir, what do you mean by 'free exchange':)?
Exchange is by definition 'John gives Mary an apple, Mary returns him an orange. John teaches her English, she pays back with teaching him Kazakh".

If you mean 'free lessons'... I know nothing about situation of this person. There are people rich and there are people poor. There are people speaking popular languages (as English).... and there are people speaking <em>rare</em> languages (there are not much of Breton learners around.. and even less of speakers).  There are people speaking popular languages (like Chinese), who are present here in numbers much exceeding the number of learners.

And there are people who enjoy teaching their language more than learning foreign ones.
"hey why don't we start with Arabic!?" "ok. BTW a have a nice Russian song for you..." Touché:(((

Why should we impose some common 'code of conduct'?:)

Exchange? Of course.

Session? Probably not.