A Theory...
If you take a look at history, I think you will find that the "lawyer loads" actually predate our current obsession with legal liability.
The majority of European (metric) cartridges have nearly ALWAYS been "underloaded" by US ammo makers, compared to European made ammo.
Not so much the British rounds, but the European ones. Done from at least the early 1900s on. Not done due to fears of legal liability, worry about blowing up "weak old guns", indeed at the time, the "weak old guns" were often still current production.
You could make a case about the change in 8mm Mauser bore size, but, even if you accept that as the reason, it ONLY applies to the 8mm Mauser.
My personal theory is that there was an unwritten "gentleman's agreement" among US ammo makers, NOT to load European calibers to the full potential.
I think this was done, to ...encourage.. US consumers to buy American. It may not actually have been the intent, but it appears to be the result. Something any good conspiracy theorist could love.
Many caliber's US loadings did approximately match the original European loads, but as the Europeans increased the loadings, US makers (generally) did not. One might consider it to have been an attempt at trade protectionism via the market, rather than tariffs.
Look at the 9mm Luger round, as one famous example. Original specs called for a 124gr bullet @ 1050fps (4"), which was changed to a 115gr @ 1150fps, shortly before WWI.
Today people expect the 9mm 115gr (high performance) loads to be in the 12-1300fps range. European makers have been doing that for some time, but US makers did not, until fairly recently.
Another interesting fact is that while we "underloaded" European ammo (or kept to the original specs) we did NOT do that with US calibers.
The most famous example of this is the .30-06 (& to a lesser degree, .308 Winchester).
The original GI M2 ball specs are for a 150gr @ 2750fps (+/-). Current commercial .30-06 Springfield ammo runs a 150 in the 2900fps range, some even a bit faster.
I have heard many differing stories about the change in 8mm bullet diameter, some stating all the rifles in German service were converted, some say they were rebarreled, some say they were not, but had the throats relieved for the larger .323 bullets.
Commercial gun makers continued to produce .318" 8mm rifles for many years after the German military change to the larger bullet. These were (and are) the market for .318 slugs, and a rather larger one, overall, than the handful of military rifles that missed getting converted, I think.