You're right - it's not predatory pricing, although I think that the question whether an airline is reacting or being proactive is not the right way to decide.
To perhaps prove my point, this is a situation that we had here in Germany:
LH runs somewhere between 14 and 18 round trips between
FRA and
MUC daily, and they have a monopoly on that route. A while back, DBA introduced flights on that route and they were a lot cheaper than
LH, but only had 5 or 6 round trips per day.
The reaction by
LH was that they undercut DBAs fares, if only by a single Euro per ticket - and put massive capacity on the route, flying 744s and 342/343s occasionally, obviously making the lowest fares much more available.
It wasn't long before a court ordered
LH to raise its fares above DBAs fares, so they increased them a bit, but retained the higher capacity.
And it didn't take long before DBA anounced that it was pulling the plug on the route - I think they flew that one for only 5 or 6 months.
In this case,
LH was only reacting to DBAs fares, it was a decision that consumers profited immensly at first - but just to complete the story - the day after the last DBA flight operated on that route,
LH went back to its original, much higher fares and to the lower capacity, even more reducing the availability of the lowest fares.
This is why I think that the reacting/proacting criteria is not the best to decide whether it's predatory or not - but in the case of these coast-to-coast routes it's not
HP going against a single big airline, but several, and I'd say that this is the point that makes it non-predatory: even if
HP should pull the route, there will still be competition between the other ones...