Not everything has to be foss, it is in company's best interest to not make it as malware. In last 20 years that I have had TS installed on my server and client, have I had it act like malware. Discord in the other hand has instantly caused issues. Not saying that TS3 doesn't have had bugs, ofc it has had.
I may not read the source code of every tool I use, but even if the average user doesn't read the source code, having it available for inspection by others in the community increases security, trust, and overall software quality. All a user really has to do is look at the license of the software they use, typically a GPL or similar license, and consider how reputable it is. Not only that, but if you're on Linux already, you can just get most of the software from your distro's repositories.
No, you should have the freedom to use it if you want. All I'm saying is that you can't really call TeamSpeak a better alternative, when they're both just as bad (they're both proprietary). If you're looking for a better alternative, consider using Mumble, it's FOSS.