I legit wonder if that was Frank just saying F it and going to commit suicide, because other than buckets of plot armor its the only reason why that plan would have had even the slightest chance of success.
For someone who routinely gives Batman a run for well prepared plans, it did seem very unusual. No armored van full of weapons. NOT EVEN BODY ARMOR!
On the other hand, here you have someone who had so much taken from him. He's killed so many, yet nothing changes. Innocent people are hurt all the time, and his pain simply doesn't subside. He tried to hide from it, only to find this new breed of criminals with badges claiming to be inspired by him- claiming to follow in his footsteps.
So maybe that was it. Maybe he wanted to show them their "hero" up close and personal. Even if he died, if he could keep these clowns from carrying out their crimes in his name, it would be worth it?
As for keeping him prisoner, well, killing Matt wasn't just about getting rid of a foe- Fisk really sought to use Matt's demise to benefit him. A hero, who succumbed to his wounds is now a martyr. And he died to save the Mayor! That sacrifice will not be forgotten!
But Frank? Frank is a trophy. For now. Just like all the others he's put in cages. I suppose Fisk planned on killing Frank after breaking him totally. Just to feed his ego. But more likely, he also plans on using the Punisher to advance his agenda. The task force was probably ordered to capture, not kill him.
A mistake, yes, and a costly one, but let's all be honest here- if Kingpin DID kill Punisher, the internet would have exploded as thousands of fans cried out in dismay. So all you can complain about is not liking the way the writers decided to not kill the guy most people didn't want killed in the first place, lol.
-Amusingly, if Frank Castle was killed permanently in the comics, I wouldn't care. I liked the idea of the character, but he really doesn't belong in a world of heroes and villains in spandex. But I really like Jon Bernthal's Punisher at the same time. Go figure.