Soaking in most springs in the Park is illegal. However, the illegality is not based on the level of danger for a given spring.
Soaking is a great idea if done carefully. There are several places in YNP where it is allowed, officially or semi-officially.
In fact, every time I visit Yellowstone NP (like 5 times a year) it strikes me how silly it is that people are kept and are happy to stay on boardwalks and paved roads. They might just as well watch the BBC documentary and save everyone the trouble. If you are outdoors you should hike, swim, wade and sense the warm thermal waters. YNP is the most regulated and civilized park I know. I think this is sad.
Driving to Yellowstone on an interstate is probably more dangerous anyway.
There are millions of people travelling to these areas every year. As this article outlines, even a few thousands of people per year travelling unrestricted in this park in its first few decades of popularity has lead to long-lasting damage of park features that we are still deprived of experiencing in their original states more than a hundred years later. Visitors today continue to pollute the pools with garbage and wander off the marked areas to 'soak' in them despite the pleas, warnings, and boardwalks meant to deter this behavior.
Allowing millions of tourists from around the world, many without any respect or knowledge of how to conduct themselves in the wild in such a way as to leave minimal impact, would lead to a rapid and permanent destruction of the very park features that make the area worthy of preservation and visiting.
Your perspective on things doesn't work in a world with 7+ billion people who are increasingly able to travel to popular destinations. The trail and boardwalk restrictions in the park make it possible for all visitors to enjoy park features in perpetuity rather than all features being destroyed by a single selfish generation.
Soaking in a hot spring seems unnecessarily risky. There's too much chance of substantial temperature change. Better to soak in a pool that's fed from a hot spring. Rather than mess with park police, find hot springs where it's allowed.
Locals know places where it is relatively safe to soak in the hot water. Heck, you can go to Chico and pay a few bucks there to soak in the same water at no risk and drink a beer..
If there's a couple million people on the whole continent, sure. But the park doesn't have the restorative capacity for everyone who goes there to do what you propose which is pretty selfish and ignorant.
If it's on the way stop by Thermopolis WY. Charming little town and soaks are encouraged. They even have several bathhouses fed by the springs and it's a wonderful experience. I especially enjoy the hot spring fed steam room.
Soaking is a great idea if done carefully. There are several places in YNP where it is allowed, officially or semi-officially.
In fact, every time I visit Yellowstone NP (like 5 times a year) it strikes me how silly it is that people are kept and are happy to stay on boardwalks and paved roads. They might just as well watch the BBC documentary and save everyone the trouble. If you are outdoors you should hike, swim, wade and sense the warm thermal waters. YNP is the most regulated and civilized park I know. I think this is sad.
Driving to Yellowstone on an interstate is probably more dangerous anyway.