The vaccines have lots of side effects. If you give something with lots of side effects and compare it to a placebo with none, people have a good guess of what group they're in and behave differently. This is often considered more reasonable than comparing to a saline injection, and it's a reason why lots of drugs now are compared to e.g. ritalin-- something relatively benign with some side effect that feels like it is "doing something".
The half-dose/full-dose regimen was a mistake in a clinic administering the trial, though. A mistake that happened to look more effective than the overall vaccine population, which is a bit troublesome and a primary reason why AstraZeneca has problems now.
Now that we have effective COVID vaccines, we can run noninferiority trials-- e.g. comparing Moderna's vaccine to a potential new vaccine-- which get around a lot of the problems here.