Looking at this particular example, I don't really have a problem with it. It's a clean, well designed page with pricing information prominently displayed and even has a mini FAQ at the bottom.
As you mentioned, getting rid of the navigation links does eliminate some distraction, and it makes for a cleaner, more focused page. It also limits the temptation to accidentally navigate away from the page before submitting the information.
That said, they might have considered making the "Assistly" logo at the top active, allowing users to click that to return to the home page, but I don't think it's really a major issue that they didn't. Similarly, they could have put a "Cancel" or "No Thanks" button next to "Begin Trial". That would make the page more usable, but I don't think it quite rises to the level of sleazy not to do it. (It's certainly not uncommon to prevent an easy escape.)
I just noticed something. It all depends how you get to the sign-up page. If you get there by clicking a button that basically says, "Sign me up!", I continue to believe the design of the page is acceptable.
However, if I get to the "sign-up" page by clicking "Pricing", now I'm not happy. I didn't navigate to this page to sign up, I navigated because I want to see pricing. I should be able to continue navigating at this point, but I can't without resorting to the Back button. I don't expect to need the Back button when navigating around a site like this, and it takes me out of my flow to have to do so. Some users might even be confounded by having to do this.
So, the crux of the problem is that the site designers have attempted to use the sign-up page for more than just sign-up (e.g., for seeing pricing), but this makes the lack of navigation buttons an actual problem, rather than just a justifiable design trade-off.