AR 6-5-2-I. Now without knowing the timing of the whistle, how many steps the 'returner' took, etc. it's hard to say if we definitely should ignore the tackle by A. I am just pointing out that this is not necessarily a late hit foul for the tackling action.
In general, I think the current wording of the rule and associated ARs paint an unclear picture of what to do when faced with a tackled fair catcher.
Note the contrast in the wording of the following citations.
6-5-5
No player of the kicking team shall tackle or block
an opponent who has completed a fair catch. Only the player making a fair catch signal has this protection (A.R. 6-5-5-I and III).
AR 6-5-2-I B1 gives a fair catch signal before a muff by B2, and then B1 catches or recovers the kick and advances. RULING: Because of B1’s signal the ball is dead where caught or recovered. Two steps are permitted to enable B1 to come to a stop or to regain balance. A third or subsequent step inbounds is subject to penalty from where the ball is caught or recovered.
If B1 is tackled, the tackle is disregarded unless deemed unnecessarily rough or is so late that the tackler should know that there was no intention to advance. If the kick is caught or recovered by Team B in the end zone, it is a touchback. If B1 is tackled before completion of a third step, only the tackler has fouled.
AR 6-5-5-II B22 makes a fair catch and is tackled before he has carried the ball more than two steps. RULING:
Foul by the tackler. Penalty—15 yards from the succeeding spot.
My sense is that the rules makers want officials to apply the interpretation as given in AR 6-5-2-I. However, both the rule itself (6-5-5) and a second official interpretation (AR 6-5-5-II) give a different, unrestricted reckoning. It seems an Exception should be inserted into 6-5-5 at the very least.
On a boarder scale, I think other aspects of fair catches should be revisited. Examples, definition of a valid fair catch signal and no advance more than 2 steps.