Though honestly, the main reason people even state difficulty in terms of "forcing partying" is because it would force SE to make the content itself difficult, since even if you get to choose the difficulty of an encounter or to solo or not, what's the likelihood of it being balanced and not ending up as going solo = cake walk, wanting to go with 2 people = a decent challenge, but going with 3 will make it so you actually need 4-8 people to actually tackle it?

This actually happens with other MMOs with difficulty choices, you may have the choice but the content design still ends up flawed on higher difficulties rather than the content just being designed for a group of x.