Just because the British Empire ended slavery doesn't mean they weren't exploiting people for economic gain- they were and continued to do so for nearly another century. The worst abuses in colonial India happened after the abolition of the slave trade. Would Americans let Britian abolish slavery anyway? The slave states revolted against even the idea of abolition in reality, and that wasn't being imposed by an imperial power across the sea. I also find it hard to believe that native Americans would have been treated any better. The US was going to expand westwards regardless. The Louisiana purchase might not have happened, but we would probably have gotten that territory anyway after Napoleons defeat. Losing the revolution wouldn't make us any less likely to exploit the West.