Proven guilty beyond all reasonable doubt?
It would be interesting to know what other evidence was presented to the jury* other than "Look! Technical stuff and some statistics!" Assuming all the States have the requirement of innocent unless proven guilty beyond all reasonable doubt, then the simple correlation of phones near incidents needs a lot more filling out** to outweigh the reasonable doubt in this case - simply being in the area at unfortunate times shouldn't be enough to get to prosecution, let alone conviction.
Oh, yes - the topic: There should definitely be a warrant for this sort of search.
* Assuming the two blokes didn't plead guilty and then try to appeal
** E.g. evidence that the phones two of only few in a place where not many people go. Being in the vicinity of three crimes in downtown $CITY would be very different from the phones showing up in the same area as three out-of-town offences, for instance.