In many cases, the Gemara rules that it is the job of beit din to offer arguments on behalf of a defendant if he does not think to offer them on his own. This idea is based on the passage in (31:8), which says petaḥ pikhah le-ilem – “open your mouth on behalf of the dumb” – if someone cannot speak, that is he does not know how to defend himself verbally, the responsible party should do so on his behalf.
The Gemara on our daf quotes this ruling in order to explain that it does not apply in the case that appears in our Mishna.
As we have learned (see daf 28), according to Jewish law, just because someone has possession of property and lives or works it, he cannot claim ownership of it. A person only becomes an owner if he receives that status from the original owner through a sale or by receiving a present, or if he claims an object that is hefker, performing a formal act of possession (a kinyan). Thus, there can be no claim of ownership based on “squatters’ rights” and if someone else comes with proof of ownership, the person living or working the land will have to prove that he bought it.
Our Mishna teaches about a case where someone approaches a person and says “what are you doing on my land?!” and the accused person responds “no one ever told me that I could not be here.” Without a claim of purchase, the accused will lose his claim to the land. The Gemara suggests that we need to teach this halakha because otherwise we may have thought that the accused person really had purchased the land, but he had lost his contract, and, as Tosafot explains – not being familiar with the law – he may have feared that if he claimed to have purchased the land, but had no contract he would lose the land and be branded a liar and a thief, as well.
The Gemara concludes, however, that in such a case we can assume that the accused would share all of his arguments with the court, so they cannot speak on his behalf.