You're right (and I don't dispute) that almost all firms aspire to long-term shareholder value creation, and that there are both practical reasons (market forces, competition) and theoretical reasons (Econ 101 arguments such as the two welfare theorems, principal-agent theory, etc. [1]) why that is and should be so.
However, that is just a broad and fast rule, and it is neither empirically true, nor desirable, nor legally required that every firm at every opportunity "maximises profit". It is this over-simplification that I argue against [0].
If Facebook is shown to pursue profit at the expense of teenager health, or at the expense of democratic institutions, or Shell is shown to pursue profit at the expense of the environment, the appropriate response is not to shrug and say "doh, that's what companies do (and must do)!".
Instead, one ought to discuss whether A) the legal/regulatory framework within which the firm operates should be tightened (eg with rules that just prohibit certain things, or internalisation of externalities), or B) whether a firm should refrain from doing certain things even though they're (still) legal and increase profits.
[0] GP didn't state, as you sensibly do, that "profit maximization should be taken to be the default", but rather (my highlights) "That's any for profit company no matter what anyone believes. Any company its only purpose is profit.", which is just the facile statement I argue against.
[1] Note that these Econ 101 arguments for the primacy of profit maximisation rest on many assumptions that don't hold, see for example James Kwak's book Economism:
Bad Economics and the Rise of Inequality.