There is a valid reason for doing so. Islam has always used violence and genocide as a tool for its growth. The existence of pre-Islamic statues remember people about those incidents. Once these monuments are removed, then in future, people will never knew that they had such a rich culture and civilization before Islam.
The only problem is such methods look out of place in the 21st century.
Just because such behaviour was acceptable hundreds of years ago doesn't mean that it can be explained / justified today.