Strictly speaking this should not be admissible in court in the US. When a warrant is issued it is supposed to outline the evidence to be seized, the crime, and the persons targeted in the raid. It could have been that the paraphernalia was out in the open and therefore did not constitute a search even though they were already in the dwelling. Unfortunately often things police just stumble upon become a bigger deal than what they showed up there for, and 3rd parties get wrapped up in lots of legal troubles as a result of someone else's actions. On a side note, where is this located? That cop looks like as if he is from the U.K.
When a warrant is issued, it is to outline both what can be seized and where the police can look. If the police are looking in places that are outlined in the warrant and stumble upon evidence of a crime they were not previously aware of then, they can investigate and arrest people for such crimes.
The moral of this is that it is advisable to not be involved with people who are doing other illegal activity