Your title is misleading. The ruling was not that computer programs can't be copyrighted, it is that copyright does not apply to the functionality of the program. If I write a program that slices, dices, and purees, and you write a program that does the same things without copying my code, you have not violated copyright. This principle has been in force in the United States for a long time. For example, IBM's bottom level code for their PCs (the BIOS) is proprietary code of IBM. But another company did a clean room implementation to clone the functionality of the BIOS (one team inspected and tested the IBM code, and wrote down what it did, and another team, that never looked at the IBM code, wrote code that carried out the same functions). There was a lawsuit, and the clean room copy was deemed legal. The cloned BIOS was sold to anybody who wanted to make PCs and thus the PC became a commodity.