Everyone's giving you the high-and-mighty ethical arguments, I'll give you the rational self-interest argument.
US internet companies need the freedom to do business with overseas clients without those clients assuming their communication will be spied on. If you start spying on foreigners using American services you hurt the providers of those services, you hurt their ability to get customers.
A compelling argument. But it's predicated on the assumption that US corporations are somehow unique in terms of the relationship they have with the local intelligence agencies, and I don't know that that's a reasonable.
US internet companies need the freedom to do business with overseas clients without those clients assuming their communication will be spied on. If you start spying on foreigners using American services you hurt the providers of those services, you hurt their ability to get customers.