Look on this other way: if you have particular stealthy troop that you don’t want to be targeted, in scenario of “one such troop” it cannot be targeted, before all other three troops will be killed.
In case it’s one such troop and three stealth one, any of your stealth troops will be targetable just after you lose your only non-stealth one. Three times faster, at least.
And here is loophole is (that became the reason of such behavior initially, I believe). Mechanically you are right, and most probably it’s how it’s implemented. But rationally it haven’t any sense: what “stealthy” in any other troop, if actually you can target any of them? In that logic Stealth must not work at all: if you targeted stealthy unit - you, obviously, cannot target any other unit simultaneously, so you always must be able to target this unit, regardless of other stealth, just with the same reasons.
All three implementations are logical, if look on it right way. 
- There are no valid targets, if only stealth units remained.
Contradiction: I believe, option “if other units exist” was implemented exactly to counter such option. So in spirit of rules, such behavior is wrong (while it still can be right mechanically).
- Units on field represents actual troops. (That’s how I look on situation). In that case they, obviously, must go in queue or in column, where only first (upper) one is close to enemy, and all others are behind him. In that representation, any stealthy unit simply hides behind other backs if targeted. I believe, some party dungeon crawlers implemented exactly this mechanic of stealth.
In such case, obviously, if no non-stealth units remained, first one must be targetable - he cannot hide behind others with the same ability.
It’s override of rules, of course. But the same override as current “everyone targetable”, actually.
- Units on field represents just that. Cards on field. They are purely abstract and haven’t any “physical” properties. In that case, of course, they can have any rules implemented (and that’s how MtG players actually answered me :). They viewed Stealth as sort of “zero aggro”. Non-stealth units have it more than zero, so can be targeted any time. But if no such units remained, you are free to select any other, equally zero aggro, targets.
Of course, it is the same artificial rule as #2. But it’s implemented already. I don’t like it, just because I don’t want to look on game units as abstract chess pieces, instead actual entities. But, as I see now (I didn’t see it initially), it’s perfectly legal approach. Only unpleasant one, in my opinion.