This is pure and simple a salary dump. Justice was due to count over $4 million against the cap each of the next two seasons and by trading him to the Colts, they're able to get that off their cap. Justice would be a backup at best here, so there was no way the Eagles were going to keep him on.
For the Colts, they get a guy who can compete for a starting job at RT. Obviously new Colts GM Ryan Grigson knows what he's getting and more or less he's gotten Justice for free. So there's not a ton of downside for him. Justice has shown flashes of being a capable RT, but health and inconsistency has caused him to get Wally Pipp'd by Todd Herremans.
The move will save the Eagles $2.334 million against the cap this season, as his signing bonus accelerates to this year and will be about $2 mil in dead money.
The team also announced that they've inked DT Antonio Dixon to a one year deal.