You must have misunderstood the news article. It didn't say they will buy what's left of FTX but they will buy or invest in cryptocurrencies because the prices are down as a result of the FTX implosion. These banks are business opportunists at the end of the day and will take a calculated risk if a chance presents itself.
Of didn't say that they were buying FTX, but crypto companies. There's no talk in the articles that they would invest in cryptocurrencies but cryptocurrency companies, and those are down because of the case FTX .
And why they do this? They see that they can get them almost free and they are in it for the long term. Many crypto companies also might have better fundamentals then most cryptos and actual value behind them. Even if they are now having hard times.