It would appear to be a partial misrendering (although likely on purpose as you say) of a more common: omnia praesumuntur rite et solemniter esse acta
"all things are presumed to have been done correctly and solemnly"
(from:
http://www.oxfordreference.com/view/...9551248-e-2695 )
or several similar phrases. The slightly shorter but similar "Omnia praesumuntur rite esse acta" shows up even more often, often in legal dictionaries, and lead eventually to here, which is interesting:
https://en.wikipedia.org/wiki/Presumption_of_regularity
Perhaps the interesting question is indeed, why the sly partial translation to French of what appears to be a stock phrase.