Yes! It is very common to wait to feed them in the morning and do training first so they are motivated. It is also common to train right before dinner time, again so they are motivated by food. Whether or not you find it cruel is a matter of how it's done (is the bird still being fed adequate amounts of food/calories?) and your personal view on this training 'method'. I personally use kisses and verbal praise as rewards for my bird with the occasional food treat. However, I train my bird because he enjoys the interaction, NOT because I particularly care if he can do tricks. He is fed a healthy diet and never denied food. He is also never 'forced' to train. When he's in the mood, I break out the props and play with him because he gets happy and excited. When he is not interested, I don't force it.
Not every bird enjoys learning tricks either. You should never buy a bird with the expectation he or she will ever learn to talk or will enjoy learning tricks. That is bringing a bird into your life for ALL THE WRONG REASONS. You buy or adopt birds for companionship and to open a loving home to them, not to give them a job.