Not really stealing
It is our money, after all, and since DoD decided there was negligible security risk and that they would allow the technology to be sold commercially, it's definitely legal.
The GPS service doesn't work like cell phones, where you need to both send and receive information. The satellites just send a precise timing signal, so all you need is a receiver that can both receive the signal and know what to do with it. The signal is already being sent for military use, so it's no extra cost to allow civillians to use it.
Because we control it, other countries can't depend completely on GPS for their military uses, which is why Europe is investing in their own system. When GPS first came out, there was an error automatically figured into the signal to prevent other countries from getting the same accuracy. US military receivers had a correction code, but civillian receivers don't. Currently this error has been removed, but if there were sufficient reason to, it could be turned on again.
I don't think any other nation has GPS guided weapons except perhaps those who buy them from the US. They could of course develop their own, but like I said, unless they play nice, we could make the signal unusable to them.