They can deny service. True.
But they have special statutory protection from being sued based on what customers post. However, once they engage in the substance (unless violent, plus a couple of specific exclusions), they become a publisher and liable for content (defamation, etc.)
Section 230 protections is a very big deal. They do not want to lose.
Yup I get that.
What I am not clear on is how the EO text being reported here actually removes those protections.