Here's the problem with the Boston Marathon attack; it was planned and conducted by US citizens in the USA without any planning with any foreigners.

When someone says "the NSA couldn't even stop the Boston Marathon bombing", they are implying that the NSA SHOULD have been able to. But for the NSA to be able to stop it, they would have to be collecting data on US citizens inside the USA.

You can't have it both ways. You can't blast the NSA for not being able to stop Boston but then also be against them doing domestic collection. The fact that they couldn't stop Boston shows that they weren't spying on Americans (even though one was on watchlists).