It is a safety issue. The billboard's whole point is you warning that if you do this stuff solo you are more likely to die. Which is unquestionably true - you are not going to be in a position to use narcan to stop the overdose if you're by yourself.

If you have 5 people who are going to use opioids, you want them to be together when they do it and you want narcan or similar available to them, if you do truly care about saving lives.