Hi Maarten,
1. Mondriaan died in 1944 and is dead for 70 years, which qualifies for public domain status in countries with a 70 years post mortem auctoris rule.
2. they claim the individual work to be public domain in the United States because it was published before January 1, 1923.
@Duke University
Yes Sir! The problem we face on Wikimedia Commons is that both US copyright and copyright of the country of origin or source country have to be taken into consideration.
Basically we don't upload new public domain content to Wikimedia Commons because it's now in the public domain in the United States, but because it has been in public domain over there already and is now also in the public domain in its source country.
As for your first question:
His works are available because there are people who upload content based on a don't care and don't want to know approach on copyright.
Even simple reality checks like "Is the work even created before 1923 and could it therefore have been published before that year?" are not done as it seems.
Mondriaan is a particularly interesting case, due to his work periods in various countries, eventually changing the country of origin or source country (that's the interesting bit for Wikimedia Commons per
https://commons.wikimedia.org/wiki/Commons:Licensing).
Additionally identifying publication status under US copyright law is not as easy as it seems:
I doubt that each and every on Mondriaan's works are made available in compliance with COM:Licensing, but there's a fair chance that several of his works can still be used.
As for your second question:
From my experience on Wikimedia Commons I'd argue that individual public domain status verification is an exception, not the rule.
Christoph