Oh my goodness, I know exactly how you feel. It's extremely frustrating, especially when the other students just get away with it.
I am in the exact same situation as you (well, I was last semester). I may have been the only one (or among very few people) who actually did the work, without looking at a solution manual (though I too had access to it). The frustrating thing is staying up late for several nights figuring out the problems and accepting the loss of points if something isn't quite right, when everyone else gets essentially a perfect. What's worse is if, like you said, the tests can't correct that.
If it's any consolation, these people, if they even make it into grad school, probably won't do well. Also, I'm noticing many of them struggle in later classes whereas the material isn't so bad for me. However, if they're in their last year, well, they may very well not do well in their jobs if they lack work ethic. Just don't let them try to drag them down with you. I make it clear I do my own work.
I didn't do any of this, but I wish I did, so here are some options:
1. Meet with the professor and inform him that this is going on. I'm sure it's okay for you to say you don't want to name names if you're uncomfortable with that. It may be the professor just doesn't know it's going on.
2. Leave an anonymous note for the professor if you're too uncomfortable to even do that.
Honestly, nothing bad can come of reporting them, if you ask me. You're obviously in the clear (especially if the professor decides to up the difficulty because of this--you'll still be fine because you're actually learning the material). Then you have to ask yourself why it would matter if anyone even found out you were the one to "tattle" on everyone else? Based on your past posts, I see you're going off to grad school anyway. Of course, this may all just be my regrets talking.
Finally, you say
Dishsoap said:
I feel as though the mature choice is just to let it happen.
In an ideal world where students lived in a vacuum, this would be true. But what happens if you need a higher grade and the professor would curve if everyone would do badly? Then the curve is lower only because the class cheated. You may not need that curve, but you never know. The mature choice is to not cheat in the first place, and they clearly didn't take that path.
If I sound frustrated, it's because I was pretty fed up with those kinds of people at the end of last semester.
EDIT. As a further elaboration on option 1, it'd be okay to tell the professor that you'd like to be anonymous. I'm sure they'd understand.
And, I will also add that the cheating students not only gain an unfair advantage over you, but also anyone else who is an honest student.