Because "by using encryption", terrorists can be in secret and plot. They don't plot "using encryption", a job that is usually done by computer engineers.
Context:
Edward Snowden ignited this debate when he revealed the extent of the US government’s mass surveillance. Privacy advocates were outraged. Tech companies like Apple and Google responded by increasing the levels of encryption in their products. Western governments hit back, saying spy programmes are needed to keep us all safe.
The potential threat of terrorism,
plotted in secret using encryption, intensifies the debate. Prior to the November attacks in Paris, both the US and UK were planning new laws to strengthen their power against encryption. Now support for these laws has grown. The debate will come to a head in the next few months as politicians finalise the proposed legislation.
More:
https://www.newscientist.com/article/mg22930542-400-how-2016s-war-on-encryption-will-change-your-way-of-life/