'Ignorantia juris haud excusat' [ignorance of the law is no excuse], for, if it were every man would claim it and no man could refute it.
But there is a difference between not knowing the law exists and not fulfiling all the requirements of an offence. Theft requires dishonesty. If the defendant claims that he though the facts to be such that he would be entitled to take something then the jury must consider a) what the defendant believed the facts to be and b) if the facts were as the defendant believed, whether he would be entitled to take the goods. If their answer is that the facts believed were such that, if they were true, it would not be theft, then the defendant was not dishonest and they must acquit.
If I take someone's umbrella in the mistaken belief that it is my own, I have fulfilled all the other requirements of theft, taking someone else's property with the intent to permanently deprive them of it, but nobody would think me a thief because, if what I believe is true, I am taking my own property. I am not being dishonest. The CPS think that it cannot be proved that this man did not genuinely believed the property could be taken in the circumstances.We have factual defences in English law, which are likely to be common to Polish law anyway. One example is if property has been abandoned permanently and intentionally by the owner and so then has no owner in law, it can be taken without that being theft.Likewise, if it has no known owner and the owner cannot be with reasonable diligence be found. If someone genuinely but mistakenly believes it is abandoned etc, and takes it, then that is not theft because they would not be taking it dishonestly.
Of course, he may not be able to repeat that argument with success !