But here you're taking it for granted that professional licensure of software engineering is actually desirable, which is not at all agreed upon. Among the organizations against the notion are the ACM.
The problem in this is that nobody agrees on this set of standard/knowledge (a Body Of Knowledge). For instance, if the document and the experts required to testify in court define that "a reasonable engineer has to use a waterfall methodology", then you could be liable for using an agile methodology (that's just an example). The domain being so young, there is not yet an agreed upon profile of what a 'reasonable software engineer' might be. The SWEBOK[1] attempt to define such a profile was initially supported by both IEEE and ACM, but - if I remember well - the ACM dropped their support in face of these concerns.
I think both organizations support the legalization of the "Software Engineering" field (and perhaps title), but also recognize that the field is not yet mature enough to be properly defined and regulated.
Disclaimer: I do not have sources for my claims on IEEE and ACM's positions on the matter; I merely read about it a few months ago. However, you can find interesting conversations on the subject by googling 'SWEBOK'.
As you indicated, ACM would constitute as (a major) part of that push back, so I don't see the contradiction.