Of course they are not as safe as a full face helmet.
This statement is generally correct, but it highly depends on the actual helmet in question. I can argue that my DOT and ECE approved modular helmet is safer than many full face ones. The best full face helmet should, at least in principle, be a little safer than the best modular one, but there is no hard data to show by how much.
Is the extra convenience of the modular helmet worth the added risk of less protection? Only you can answer that question. Like most motorcycle safety discussions it all comes down to risk vs reward...
Safety is always a question of trade-off, but added convenience can sometimes directly translate to added safety. In my case, for example, the fact that I can blow my nose without taking the helmet off is adding to my safety.
If they pass the same testing as normal full face helmets, how are they any less safe?
They don't. Snell tests only full face helmets. There is a lot of controversy with regards to correlation between helmet testing and real life safety. The fact that one helmet has Snell sticker while the other one has only DOT does not prove anything except that each satisfied that particular testing standard.
In the absence of hard data it is mostly guessing, although it's based on some pretty solid logic. A hinge, no matter how strong, will always be easier to break than a solid piece. I've seen too many really pathetic modular helmet joints to discard that factor as marginal. However, the chin bar coming off opens ones head to impact only partially.
All in all, the jury is still out and will be until some hew Hurt comes up with a proper report. I have chosen modular a while ago and don't plan on going back to full face.