I think the DM is this case is trying to play the wrong game. If the Tank is doing everything he can to pull agro and the dm ignores the fact that that is his hole purpose in the game and focuses on the strikers just because, then he's just being evil. This game is designed around combat roles, if you are just going to ignore them, i suggest all of your players roll defenders or they're all going to die anyway.
It depends on your group.
Many of my players actually enjoy having their PCs attacked, regardless of role. It allows them to participate in ways other than "I hide in the back and snipe".
I have one player who every single time an attack hits his Warlock, he says "Cool.". Whether or not he means that he likes it when his PC is hit, or whether he enjoys taking his fair share of the load, or whether it's just a habit, I cannot say.
But I probably wouldn't stay long in a game where the DM mostly targets the defender. How boring is that? There are 5 PCs, that means that there are 5 legitimate targets and it's up to the players to protect the PCs that are less durable or who are more injured. That's not the DM's job.
The DM's job is to challenge the players and if there is a bloodied PC on the board, it doesn't matter who it is. Most NPCs in my game that can safely attack that PC will do so like sharks on piece of meat (NPC dependent, stupid zombies tend to attack the closest foes, only semi-intelligent NPCs pay attention to things like the bloodied condition or disadvantaged PCs).
We even had a game where the PCs were running out of time and the Fighter was totally out of healing surges just before a major battle. So, they put the Fighter in the back and gave him javelins to throw. He didn't go unconscious in round three because he wasn't up front taking a lot of hits, and was able to participate in the entire lengthy encounter.
We've had a lot of encounters over the years where one of the high defense PCs who were almost out of healing surges were shifted to the back of the group and lower defense PCs with a ton of healing surges were shifted closer to the front. That way, the team could handle more encounters that day. Yes, the party might be able to use Comrade's Succor and keep the high defense PCs in the front, but sometimes, that's not the best option (e.g. the PCs don't have enough time, or the group is really low on healing surges, has to do the ritual two or more times for two or more PCs, and they don't want to waste 2+ healing surges, or they just don't have the ritual, or they don't have a ritual caster, etc.).