Tyger hit on this first, and he is right. The testing for the UV light was done for VERY CLEAN WATER save the target test organism, sans silt and other particles to allow for maximum UV penetration. How many people have seen that clear of water on trail???
Even dead organisms (bacterial, viral, amoebic,etc) can still harm you after they are killed. Many have toxins that your body reacts to, and those toxins remain in the organism even in death. Read, you can still get sick, although the continued production of the toxin has been stopped and you will most likely get over it.
This really holds true for any chemical or light treatments. Only a filter will actually clean out actual particles, be it a ceramic, charcoal, or other. The smaller the pore size, the better (0.2 micron is the best). But, also, the smaller pore sizes clog up faster and can be a pain.
On that note, I use a bandanna to particle filter out the big stuff and PolarPure to kill. My personal preference, risk vs weight factors for me. haven’t gotten sick yet.
If you question the water source, treat it. If you really question the water source, move on. Other hikers, guidebooks, and locals can often help point you to a good source if needs be.
And always remember to carry a little 4-packet of loperamide, just in case (weight= 0.05 ozs, savings in TP, pounds!!!). 
-xtn :boy
airferret