It doesn't surprise me in the least. AA already won't check bags across tickets although it is an easy task.

As UA trains fewer agents on SHARES and forces AERO, this becomes a time to end "work arounds" which it does not want.

There are plenty of reasons not to check bags across tickets. Not the least of which is that every connection increases the odds of loss or damage and therefore liability. That is part of the risk a carrier assumes when it sells connections. But, these aren't connections.