It can be if done the proper way.  You're not supposed to just flat out sit on the saddle itself.  In trot you're supposed to put the weight on the stirrups instead of just sitting there.  Here's an example.  Forgive the shitty video but it'll demonstrate how you're supposed to ride. However people who either don't have the strength or are just to heavy for themselves will not ride properly and put all their weight.  Here's an example of that