What's wrong in that? You are playing at the highest level of cricket and you must not expect any thing easy from the opposition just because you are a mug.
To me, what is acceptable depends on the "batsman's" level of ability. In Martin's case, that level is pretty much non-existent. This has two effects. First, he cannot protect himself. Second, there is simply no need to bounce him out. He's not a Hoggard-type of player who is basically unable to score but can block the straight ones; he's a player for whom the fast, full, straight ball will consistently get him out virtually immediately. Therefore Anderson's bouncers were completely unnecessary.
To take another example: Courtney Walsh subjecting Devon Malcolm to a very nasty onslaught of short pitched bowling. Malcolm was a desperately poor batsman, albeit marginally better than Martin, and what Walsh did was in my view wrong.
To be fair, I didn't see or hear Martin or any of the other NZers complaining even when he was (IIRC) hit. It made for extremely uncomfortable viewing though.