This is normal accounting practice. Not only is it legal, it's essentially legally required for companies to track the changes in the value of their assets. I

This is normal accounting practice. Not only is it legal, it's essentially legally required for companies to track the changes in the value of their assets. It matters more in the downward direction, a company has to know when they are insolvent (owe more money than assets they hold) because they are required to file for bankruptcy. Value of an asset goes down, they need to update the books and make sure they don't have too much debt. The only real issue, are the valuations accurate? If they're not, then it might be accounting fraud.