Yes, technology companies should be allowed to alert users when they're being spied on. Honestly, everyone should assume they're being spied on. But, that doesn't mean they shouldn't be informed. A person using their own personal technology should have a right to assume privacy. If they are being spied on, they should be informed.
If there is spying going on it is only fair that you should know when you are being monitored. At least there should be a visible camera or other object that alerts you to the fact you are being watched. I think it should go further as well, to monitor audio spying.
... especially without them knowing it. Even the NSA needs to make sure the citizens know the extent of their spying, and the Congress shouldn't be lying about this, the way it has been. Edward Snowden was a hero, spending time in a foreign country now for the sake of his own nation, that has outcast him.
Yes, technology companies should be allowed to alert users when they are being spied on. I believe that personal privacy is very important. When you take away a citizen's privacy, they lose trust in their government. While spying on users could be an advantage for the government, the problems it causes would outweigh the benefits.