Synaptic allows installation of non-free software

Synaptic installs whatever is in the repositories you've configured Trisquel to use. So in other words they must've changed the repos manually. You won't find proprietary software in the official repos.

If somebody wants to install proprietary software on Trisquel, that's a silly thing to do as it completely misses the point of this great distro. But preventing people from doing so is impossible and would make Trisquel non-free.

The internet is full of bad advice and clueless people. You just found another one. (google's blogspot in the URL made my spider-sense tingle even before I clicked the link...)

I think someone should discard this non issue (as that's what I think).

I agree. This is not an issue for us. The default is freedom which I believe is the distribution's responsibility, along with not recommending or assisting with non-freedom. Changing where synaptic points is the user's responsibility.

Besides, I believe the philosophy of this project has been that if the user chooses to "do the act" of installing non-free software themselves, then it should work. The caveat is that we will not help them or recommend it.

I think people interested in non-free software can already install any number of other distributions which will contain it by default.