Why are coin miners (such as GUIMiner) a "threat" in Windows Defender? I opened up my GUIMiner and I got a rather unfriendly "Win32/CoinMiner" threat of "Medium."

I installed this program knowingly, so should I be concerned? Is this just a problem because people have been installing miners on people's machine unknowingly (which would be a bad thing)?
It is because of hackers that bundle the CPU miners in their viruses causing unaware AV makers to ban the miners that are being used. (including cgminer due to the fact it was cpuminer)

EDIT: According to nmat, there are also virus GPU miners.