Nicotine is a poison. So, proper labeling/packaging is required. The warning is just a standard warning required by law (U.K. -- other markets will likely have similar requirements) to meet safety standards, etc.
If you
vape enough
e-liquid you can get nicotine poisoning, so it's simply a fact that
e-liquid (containing nicotine) is toxic by inhalation. Just that most of us never vape enough, or enough of a high nic content liquid, to have serious health effects. Most know when they've had too much so put down the e-cig and recover before anything serious happens in terms of health.