Also, many solos might simply drop the fighter right there. If he's a problem, then focus all your attacks on him and put him in death-check-o-slavia. This isn't always possible, but it's one alternative the fighter just might not like.
It's hardly RAW, but I often swap-in a forced move power for brutes, soldiers, etc. It's just a little bit more fun to be thrown around than it is to be dazed, stunned, immobilized, etc.
So, for example, a breath weapon on a soldier might do damage + push a target 3 squares, rather than do damage + dazed (or whatever).
I had a solo designed with pushing as a defense and then met Form of the Willow Sentinel from the Warden, which negates all forced movement. The most insulting part being it was a pregen I built myself.
The one time I used this power with my own fighter I found myself pushed out of reach the following round. The following day I was unable to get into a good position for it, and by the next chance I got I had retrained it at 6th...
This happened in my first 5th level solo fight. The dwarf used Pinning Strike fairly early in the fight and there was NOTHING i could do per the rules to let the solo escape. Nothing. It was somewhat annoying.
But answer this if you can: if the solo had an ally, and the dwarf attacked that ally without moving, then what is still pinning the solo down?
As a note... 'just eating the fighter' is not really a valid response. A fighter backed up by any leader can probably hold off most solos until the party kills it, especially one that is unable to use special tricks, is forced to go after the person who has special bonuses on them (defensive stance, shield of faith, battlerager, what have you).
The fighter /is/ supposed to be the Defender. If this power encourages the enemy to concentrate his attacks on the fighter, and that makes the party's victory easier and more certain, then it's certainly a good power for a defender. The Imobilized conditions are handed out by other 5th level Daily powers (Web over a large area, for instance), so it hardly seems out of line.
Besides, it was already tough to move away from the fighter, to begin with. He gets an Immediate Interrupt if you shift away, and his OA if you move away spoils your move.