Portal 64 uses a proprietary library by Nintendo (namely libultra). This library cannot be freely used, and Nintendo can sue anyone for using it without their permission. Portal is Valve’s intellectual property, so there’s a risk that Valve can be dragged in into this if Nintendo takes legal action.
Valve doesn’t want to risk dealing with Nintendo’s lawyers, so to be on the safe side they ask for Portal 64 to be taken down instead. The risks might be slim, but Valve don’t want to take any chances with Nintendo.
Portal 64 uses a proprietary library by Nintendo (namely libultra). This library cannot be freely used, and Nintendo can sue anyone for using it without their permission. Portal is Valve’s intellectual property, so there’s a risk that Valve can be dragged in into this if Nintendo takes legal action.
Valve doesn’t want to risk dealing with Nintendo’s lawyers, so to be on the safe side they ask for Portal 64 to be taken down instead. The risks might be slim, but Valve don’t want to take any chances with Nintendo.