Spam filtering works using actual data that someone once used to train it - if we are taking this at face value, it means that the NSA is not just targeting people who might be terrorists and collects data on other people as a side effect, but that it actively targets people with no ties to terrorism whatsoever, for reasons that both US and international public might find unsavory if they found out about them.
Spam filtering also works in a very different context - the spam-to-nonspam ratio is something like 90% spam and 10% nonspam, which means that there is lots and lots of spam to filter out; if an important email slips through, people are bound to notice and either adjust their spam filter or do something about it.
In the other setting, you have 99.99% or more of people who have nothing to do with terrorism or criminal activities, and maybe one or two dozen (among tens of millions) who you are actually targeting. First thing, erroneously targeting a substantial chunk of your non-interesting population ties up resources - you're spending your time investigating people who are not terrorists - but since it's difficult anyways, at least you seem like you're doing something with all the money you receive, and nevermind if some of the data is used for industrial espionnage or hunting people that only poultry farmers and fracking magnates would call terrorists. And if you miss one of the two dozen other people, well, they won't do anything harmful this year or the next because they also have to fear regular law enforcement, and when they do it'll be in a moment that's probably suitable for you to ask for more money.
tl;dr: Because we don't have a large sample of actual terrorists on hand, it's hard to evaluate activities like the NSA's, which would however be desirable since we're giving large chunks of money to them that could be fruitfully used in making everyone safer if used to fight actual crime and not some fuzzy notion of terrorism.
Spam filtering also works in a very different context - the spam-to-nonspam ratio is something like 90% spam and 10% nonspam, which means that there is lots and lots of spam to filter out; if an important email slips through, people are bound to notice and either adjust their spam filter or do something about it. In the other setting, you have 99.99% or more of people who have nothing to do with terrorism or criminal activities, and maybe one or two dozen (among tens of millions) who you are actually targeting. First thing, erroneously targeting a substantial chunk of your non-interesting population ties up resources - you're spending your time investigating people who are not terrorists - but since it's difficult anyways, at least you seem like you're doing something with all the money you receive, and nevermind if some of the data is used for industrial espionnage or hunting people that only poultry farmers and fracking magnates would call terrorists. And if you miss one of the two dozen other people, well, they won't do anything harmful this year or the next because they also have to fear regular law enforcement, and when they do it'll be in a moment that's probably suitable for you to ask for more money.
tl;dr: Because we don't have a large sample of actual terrorists on hand, it's hard to evaluate activities like the NSA's, which would however be desirable since we're giving large chunks of money to them that could be fruitfully used in making everyone safer if used to fight actual crime and not some fuzzy notion of terrorism.