It's not only teamviewer. They used to use logmein too, until they stopped free accounts. There's another tool that starts w/ an S (splashtop?) idk but my mother in law just got hit w/ that one... Payed 250 to hand over the keys to her machine and then turn off her AV and firewalls have them download the free version of Norton from Comcast which interestingly didn't detect their malware, and installed malware to keep it from being turned on for anything but on demand scans. Luckily MWB found it. I'm in the process of gathering her data off that drive pre-nuke later today.
Classic "We're from Microsoft" scam, certainly not the tools fault! Could be done just as easily with any other client. Personally, I find Teamviewer to be an awesome workaround for me.