All I can say in response to "what are my opponents thinking" is: target priority. You only need one more objective than your opponent to win 2/3 of standard games. All I have to do is hold one objective -- with any one of 3 units (6 FWs, 2x Kroot/Hound) -- and merely kill off everything else (or contest it) and I win. Most of my wins with that min-maxed army were 1-0 or 2-0. Close, but a win is a win is a win. And holding and keeping that one critical objective proved surprisingly easy when the enemy had to prioritize targets.
If you're mech -- and in 5th edition your really should be (all my opponents are) -- your army depends on the transports and heavy tanks. If you lose those, you're exposed and the Tau chew you up. That meant that even if they wanted to shoot at my troops -- even assuming they were in a good position to do so -- it is going to be smarter to attempt targeting Tau units that threaten the mech army's method of obtaining victory. Nearly every player will play to protect their method of victory first, and that will almost always mean targeting Tau battlesuits and anything with a railgun. Especially if you're running, say, 12 crisis suits, 4 broadsides, and a hammerhead. Mech armies hate that kind of army, especially when there are also 4 piranhas and a devilfish blocking your rhinos and land raiders from getting close enough to smash the faces in of any dismounted FWs (rare) or Kroot (more likely).
Armies like IG depend on their armour to win games, too, so they also prioritize my armour and battlesuits much higher than the Troops. They have to if they want to play to win. (Granted, I haven't yet faced the new IG, but, having seen many of the army lists floating around ... I'm a little wary. They may prove to be the best army out there. Scary, scary stuff. So many viable builds, so much speed and long-range power.... Looking forward to seeing what my first matchup will be like.)
The key to winning such battles with my style of play is to control the board space. Tau can do this exceptionally well, much better than almost any other 40K army, IMHO. If the board space is controlled properly, the enemy really isn't given a choice to do what you're recommending. It wouldn't be prioritizing threats to their army properly. Not if they actually wish to WIN. At best, they MAY be able to force a draw out of me. (Certainly no guarantee they can get all my Troops dead, not with going to ground in cover and with screening them heavily with skimmers, etc.) But they're so afraid of getting tabled (auto win even in objectives missions, and this is a real possibility with a min-maxed Tau army) that they are going to play to their strengths in an attempt to actually win rather than be so conservative as to hope to keep at least one model alive and say, "Ha! Draw!"
This probably won't satisfy you as an answer, but I don't have a better one to offer. I guess ... either it seems plausible to you, or it doesn't. Either way, it works out just fine in actual game play.