One man's "activist" is another man's terrorist, basically. Activists are liable to convert to extremism with the right set of triggers. This can therefore qualify as "counterterrorism", because if they access all your communications they may be able to flag you as a potential clinic bomber, etc.

Also, I seriously doubt the assertion in the headline that terrorists simply don't use common online services, and even if this were true, the ripples that can be correlated from their associates who do use these services would still be of interest to law enforcement.

Depends on your end goal. If the goal is actually what's stated, I.e. stopping terrorists, there is no value.

If the actual goal is to produce a massive database of citizen activities for later (ab)use, then there is a huge benefit for the government.

Since all of the evidence seems to point to scenario #2 (since there is no good reason that tracking those eeeevul turrists) requires all this information from innocent people, I think we can safely assume their excuse is a shitty facade.