Iunderstand why people find this strange and unfair, but there really wasn't another way to accomplish what they wanted.
Can you imagine if any nation, or say the UN, suddenly decided to ban something like fossil fuels? All of a sudden an important commodity, from a financial perspective, would be worth nothing (at least legally.) Companies that had invested significant amounts in research, discovering and exploiting oil reserves, even developing products, or petrol stations, would find their major assets worth nothing.
There would be bankruptcies all over the place and probably a huge market crash.
With the end of slavery it wasn't as all encompassing, but it would have had significant implications for the economic stability of some of the colonies had there been no effort to compensate the owners, as well as implications for actual production of things like agricultural products. Total economy crash wouldn't have been good for anyone.
There were some attempts to try and make sure former slaves weren't just kicked out and left to fend for themselves, which didn't work particularly well. However, this was the first time something like that had been tried, and some of the solutions we might use today either weren't thought of then, or the infrastructure didn't exist. Modern land redistribution as has been tried in a few places wasn't something being done at that point (and it's not like it's ever worked that well anyway.)