How the police can find about these transactions?
Even if is hard to track illegal transactions, I don't think that Bitcoin can be a danger to society, since the problem is the society and its people.
Since the beginning of the internet this is happening ( the bad people committing crimes) and no one is willing to shutdown internet because of that, so in that matters Bitcoin should be equivalent. I think those illegal transaction are one of the reasons some governments and the media are trying to depreciate bitcoin, but the problem is not Crypto/blockchain or Bitcoin, is just the society.
There are so many beqs that are saying that the butcoins are being used by the criminals but there are still a rumor and not yet proven. But what I have heard is they are using the digital currency of course they are untraceable and very convenient when it becomes into global transaction and the criminals are liking it but still the bitcoins are not invented to be like that but instead the people is the one who choose to be that way but lets not all do that.