"X was Y" doesn't necessarily imply that X is no longer Y: for that, English has "X used to be Y" construct.

"is" would be so much more descriptive in this case that one could argue the usage of "was" may be intentionally confusing, as to induce the reader to imagine either the surveillance ended or became legal.

'is illegal' wouldn't communicate that it was illegal at the time. It might be taken to suggest it is presently illegal but wasn't when it was done.

"I used to do drugs. I still do, but I used to, too."

- Mitch Hedberg

It was recently found to be unconstitutional by the supreme court, no? This seems to be asserting a stronger claim that it was also illegal then, not that it has since ceased.