Personally, I don't really see why so man get upset by it. The horse doesn't seem scared, or look like he's breaking his back by the weight.I rather find the act rather sweet and amazing; how it's possible to get two natural enemies to work together.
So the act in itself seem ok for me.. especially if the horse had been a little bigger and more muscular.
But there might be other things that makes it cruel; how is it trained? With patience and calm, or with whips and straps?
Has the lion had it's teeth and claws removed? How nice is that? (claws cut is one thing.. but pulled out? And teeth?)
How about the safety? Are they sure the lion won't attack (if it had teeth left)?
How long time anf how often does the horse carry that weight? Is it trained to be srong enough for it?
And the horse could need more food and more muscle, and there are certainly more things that's not alright.
But what do you think? Is it animal cruelty just to have a lion riding a horse like this? In that case; why?