That term doesn't make sense. Streaming a video involves transferring portions of it to your computer. Some portion of the video has to be present in memory on

That term doesn't make sense. Streaming a video involves transferring portions of it to your computer. Some portion of the video has to be present in memory on my computer in order for me to see/hear it, which means that the act of watching a video necessitates making at least a partial copy. You could claim that there's a difference between holding the data in RAM and holding it on disk, but all modern consumer operating systems support virtual memory, which means they can use the hard drive as an extension of RAM if needed. That would mean that it violates the terms of service to watch a video while doing something memory intensive, which could cause portions of the video to be automatically transferred to disk. I'm not a lawyer, but I question whether this term could be enforceable.