Why does Microsoft advise against readonly fields with mutable values?

It seems natural that if a field is readonly, you would expect to not be able to change the value or anything having to do with it. If I knew that Bar was a readonly field of Foo, I could obviously not say

Foo foo = new Foo();
foo.Bar = new Baz();

But I can get away with saying

foo.Bar.Name = "Blah";

If the object backing Bar is, in fact, mutable. Microsoft is simply recommending against that subtle, counterintuitive behavior by suggesting that readonly fields be backed by immutable objects.