Why it is determined that Katniss needs to be out in the field instead of on a sound stage to be filmed for the propos?

Haymitch thinks that Katniss performs better in "real-life circumstances", thus, they decide she should be filmed in actual combat.

"Well, that's all very nice but not very helpful," says Fulvia peevishly. "Unfortunately, her opportunities for being wonderful are rather limited here in Thirteen. So unless you're suggesting we toss her into the middle of combat--"

"That's exactly what I'm suggesting," says Haymitch. "Put her out in the field and just keep the cameras rolling."

"But people think she's pregnant," Gale points out.

"We'll spread the word that she lost the baby from the electrical shock in the arena," Plutarch replies. "Very sad. Very unfortunate."

The idea of sending me into combat is controversial. But Haymitch has a pretty tight case. If I perform well only in real-life circumstances, then into them I should go. "Every time we coach her or give her lines, the best we can hope for is okay. It has to come from her. That's what people are responding to."