Shouldn't be the questions be trickier?
Don't you think the type of questions "Mark all correct translations" should be tricker? For example, when I translate sentence "A dog walks towards the cat." there is only one translation containig "Un perro", so I don't even think about a correct form of the verb. Is this going to change in more difficult lessons?
I'm up to level 15 and they are starting to be a tiny, little bit more tricky. But I believe there is a reason why the choices are made so easy. In dog training, there is a fairly new school of thought that if you make it nearly impossible for the dog to do the wrong thing in a training session, the dog learns faster and everybody feels successful. I wouldn't be surprised if this same theory works for people. The idea is that if we see the nearly-right (wrong) translation, that translation would be reinforced in our minds, so we avoid that and only reinforce the right translation.
I can't see much point in them unless they are trying to explain a particular grammar point. I would prefer a short explanation if there is a bit of a tricky language involved.