Setting the human rights issues aside, here's a compelling practical reason: spying on other countries can be a side-step towards domestic spying. By spying on

Setting the human rights issues aside, here's a compelling practical reason: spying on other countries can be a side-step towards domestic spying. By spying on China, we can also capture any data they've captured on us, without circumventing any laws prohibiting spying on citizens. It wouldn't take long before this devolved to backdoor deals with allies; we spy on Germany, Germany spies on us, and we each "accidentally" get to scoop up data on our own citizens. (Of course, this is already happening now; they just don't have to lean on it as heavily, since most of the countries in question are spying domestically with impunity.)