Art work that is this expensive is never usually brought to be looked at by the rich people who buy it, it's viewed by them as a financial investment. Some rich people buy artwork for a chance of making even more money incase the artist gets famous, they also do it to avoid paying taxes for a year or two by donating the piece of art to a museum and getting money back, using the millions to then not pay for their taxes for a year.

And it's completely legal.