Your weed-buying friends are quite safe from the full power of the NSA and 'parallel construction' being brought to bear on them and their petty crimes. I can't really understand why anyone would even _think_ that would be the case?
In fact, I just don't understand the problem people have with the 'parallel construction' of evidence. By its very nature, if the original evidence existed, and new alternative sources can be found, these it must be believed that they point to the fact a crime has been committed. In which case it should be prosecuted, and the criminals brought to justice. Or are people suggesting that if the NSA finds evidence of a crime, that crime can no longer be prosecuted even using alternative, non-NSA sourced evidence? That we should just wait until investigators stumble across the crime by accident, and allow many criminals to avoid prosecution?
My feeling is that if a valid and uncontested, un-tainted evidence trail exists, showing that a crime has been committed, then morally there is no problem. We are not talking about the use of torture to obtain the original evidence, for example, rather it is a side effect of the intelligence gathering process. This sort of cross-pollination between intelligence agencies and law enforcement has always been present, and the source of the original 'tip off' was generally inadmissible due to classification or to protect sources.
This is not even a new idea. Law enforcement has to deal with 'tainted' and inadmissible evidence all the time, from protected sources, conversations overheard in bars and so on... Sometimes the evidence has even been allowable at the time it was collected, but circumstances change later. Anyway, in these situations it is standard practice to try and obtain different, non-tainted and admissible evidence pointing to the same outcome.
We now have a new word for this, in 'parallel construction' but the outcome is no different.