Why can there only be PET bottles with a Swedish deposit mark in the PET bags?

Apart from the cans, which almost solely are made of aluminium (there are just a few metal cans) , we can't ensure that the plastic in foreign plastic bottles or bottles without deposit are put together with the proper type of plastic for us to be able to recycle them in our system. A mixture of the wrong sorts of plastic can ruin the recycling process and the material can therefore not be used for making new PET bottles.