Why is it weird that journalists have to sign an NDA to visit premises? Your job is explicitly to spread information and you're visiting a commercially sensitive area. I've worked with small companies (< 30 people) where entire floors were off limits if you didn't sign.
It's easy for a journalist to describe something without realising its importance within the community. There was an article recently about a tour of a Nikon lens factory and they were forbidden to talk or describe certain furnaces. If you weren't an expert in glass manufacture, you might let slip a key fact like "we saw a big machine before process Y" or "the glass at this point is red hot" (maybe everyone else has it orange-hot).
Why should any corporation let you visit their premises without suitable assurances?