>Presumably the reason they tell us specifically about changes is so that we can be prepared for writing code that allows us to avoid problems due to changes. Why isn't it just as important for us to be prepared for writing code that allows us to avoid problems due to already known bugs?

It's obviously very important for us to avoid problems due to bugs in VFP. MSFT definitely publishes (some) known bugs, but it appears that this only applies to non-current products. As to why it's "not important" to MSFT to publish known bugs in current products, I don't know. I vaguely recall discussions like this in the past. Maybe a search will bring up some information.

>I am having a lot of trouble seeing the reasons for the difference in attitude between the two.

There's obviously a difference in practice between announcing new product behaviors and announcing known bugs. Determining another's attitude is subjective and I don't care to speculate.