TAILS is a distribution made for anonymity and security. One that forces all traffic to go through TOR. This is majorly different to most other Linux distributions. It is unique, and questions related to it are likely to be more specialized.
I mean, on-top of the tags we have for more popular distributions like ubuntu, and debian, we also have tags for lesser known distributions like solaris and netbsd. I believe TAILS deserves it's own tag as much as any other distribution does.
According to DistroWatch, in the last 12 months, TAILS has the 35th most "hits per day" (on average) of the distributions on their site. It is two distributions below pc-bsd (which as you can see, has it's own tag), and six below redhat. This shows that it is fairly well-known distribution, and therefore has the potential for people to benefit from questions asked about it here.
To further prove my point, below are a few examples of previous questions that should be grouped under the TAILS tag:
And finally, I also believe that if there were to be a tag for TAILS, more people would ask (and therefore answer) questions regarding it (benefiting everyone in their community).