If Google can't protect its datacenter traffic from the NSA, or from the law, what makes you think DDG can operate outside the law or protect itself from NSA snooping?
DDG could be forced to put an upstream intercept which would log searches by specific IPs, but that's outside their normal operational scope. It also applies only to data-on-the-wire, not data-at-rest. In Google's case, your aggregated history is sitting on their servers to be collected at any time.
If you don't trust DDG not to log your searches, going through Tor will give you yet another degree of separation (there are also browser extensions such as DisconnectSearch: https://disconnect.me/search).
As sobering as the disclosures from Snowden, Applebaum, and others have been, the news appears to be that cryptographic methods do work to protect privacy or at the very least greatly increase workload for surveillance.
And another aspect is that by encouraging and promoting alternatives to Google, we're carving out at least a small niche in which privacy-focused entrants to a vastly concentrated search market (Google scores in the neighborhood of 85% of all search according to some metrics I've seen) might be able to thrive.
DDG's safety from the NSA is inversely proportional to its success in the market.
DDG doesn't operate that way. Access it through a sufficient diversionary proxy (Disconnect Search, TOR, what have you), and you're simply another (unknown) IP address making another (known) search request. While I won't say it's impossible to tie the two together, the cost is far higher than the case where 1) user search history is explicitly stored or 2) a direct IP history is connected.
Scale your countermeasures to suit your paranoia level / risk model. If you're concerned over browser fingerprinting (https://panopticlick.eff.org/), you'll want to include privoxy as well as TOR.
Note that even unauthenticated users to Google are issued cookies, and undetermined amounts of browser state are tracked. I've got a statement from a Google engineer on G+ that such indicators aren't used to identify accounts, but whether or not they're used to identify end users at all is an unanswered question.
So, short answer: you're right, using DDG of itself isn't a perfect guarantee, but it's a much smaller risk envelope than Google offers, and it can be reduced to pretty near nil with a few additional provisions, all of which hugely increase intercept costs.
So if you're using Google directly via TOR, you're back in the risk case that your data-at-rest identify you. They can be linked by various means: your username, if that identifies you, by patterns of behavior across multiple sites, by ad syndication networks and shared cookies, etc.
So, no, TOR alone isn't sufficient security in the case of Google.
If the NSA is tracking you across the internet outside of Google's Search Box, you've got bigger problems than Search History. Searching for "Bomb Making 101" is the least of your problems if they see you actually visiting BombMaking.com as well as BombMaterialsShop.com
You're postulating a threat model in which anonymized and proxied browsing isn't good enough, because somehow they'll capture all your behavior anyway and then tie it to anonymous search history. My point is, if they can do that, frankly, the fact that they have your search history is the least of your troubles.
This appears to be a threat model specifically designed to sell the DDG use case and fight the notion that using Google un-logged in via a privacy browser isn't "good enough". I'm not sure the case can be made that the DDG scenario is marginally better enough to justify worse search results.
It's not just a question of NSA snooping, some of us simply do not want our online behaviour tracked and recorded by companies to such an invasive degree.
DDG does not know when I print to my desktop printer or how often (but Google does if I'm using ChromeOS).
DDG does not know my journey within different websites, but Google does via it's analytics service.
DDG does not stitch together seemingly disparate journeys that I make online (e.g. browsing a product on a shopping site that uses Google Anlaytics, then going to YouTube to search for said product - Google undoubtedly connects these journeys)
DDG does not record every website I visit, every link I click. Google does if I'm using ChromeOS or if I'm signed into a Google account and using the Chrome browser.
DDG can not track my online behvaviour across desktop (Chrome and ChromeOS), mobile (Android) and tablet (Android) to the degree that Google can.
The extent to which Google can track and record online behaviour simply has no equal among online companies. No company that collects such a staggering amount of data about users should be free from scrutiny.
And finally, it's also about taking a principled stand rather than just shrugging your shoulders and thinking "Well, the NSA will snoop on me whatever service I use, so I'll just stick with Google even if I don't like their online practices"