I'm a Project Manager for a large, comprehensive cancer center in the US. They actually do have placebo arms. Not always, but a lot of them do, once they get into Phase 2/3.
They're used if there isn't a standard treatment established to give to a patient. Or, more recently a lot of multi drug trials/multi modality trials will have placebo arms to compare more than 1 treatment, against each other or against what standard treatment is already FDA approved. Language is typically: 'Pt will get Drug 1 & Drug 2, but for Drug 3, they will be randomized to Drug 3 or placebo.' OR 'Pt will be randomized to Standard Treatment or Drug 1 or Drug 1/Placebo.