I may be mistaken, but I don't believe the article said anything about convictions. The 42 "terrorism suspects" had their call records etc. sent to the NSA under court order. There isn't even mention of a trial. Was there due process? Was they considered innocent after reading the call record data? We don't know anything other than the NSA claims some people were "terrorist suspects", the FISA court granted a warrant, and phone records were collected.
> who were likely to commit violent acts at some point
That's an unwarranted assumption. All we know is some people were labeled a "terrorist suspect". Even if the investigation was for a violent act, they are suspects; no guilt has been established.
A larger problem is your assumption that "terrorism" implies "likely to commit violent acts". That hasn't been true for some time. Drug trafficking[1] and the Occupy protests[2] were both "terrorism" according to the FBI. The term "terrorism" was diluted down to meaning "crime some people find frightening". The TLAs tend to use it now mostly for funding/budget purposes, as yet another "think of children!".
[1] https://archives.fbi.gov/archives/news/testimony/internation...
[2] http://www.huffingtonpost.com/2012/12/23/fbi-occupy-wall-str...