That was in 1975. The NSA that did that isn't necessarily the NSA that exists today. Just because they did something good nearly 40 years ago, dosn't mean they have anywhere near the same ideas now.
Internal priorities, people with the ability to push their agenda, and external factors can have easily changed in that time. Hell, most of the people from back then are probably dead by now.
Also, while they did make it stronger against differential cryptanalysis, they got the key length reduced, which means that today, DES is terribly weak, and 3DES is needed to patch it up.
This fits in quite nicely in what you say though. The thinking might have been that differential cryptanalysis makes cracking much easier, but a reduced key length would still require NSA-sized resources to break.