His units are worth double because the player that was playing with him left, not because someone else joined the game.
His units were worth 350, but then the other player left and forfeited his 350 points worth of units, effectively giving them to the other player.
Once the CD player has left the remaining player effectively has units worth 700 points, and it's only fair that newcomers have to put forward the full 700 point bet.
Or, in the earlier example, each player put 100 points into the pot, and has an equal number of units, and then a player leaves.
Now there is 700 in the pot with only 6 players, each player effectively gets a slice of the CD player's bet, so each player's units are now worth ~117 instead of 100. It's only fair that a newcomer to the game also has to pay 117 to get units worth 117.
The newcomer doesn't increase the value of the other players units by joining the game. If a player leaves the other players get the exact same increase in value per unit whether or not another player joins.
Then, when another player joins that player puts money into the pot, but the player is also bringing units into play that were no longer in play, so the effect of increasing the pot size cancels out.
If a pot is 700 in size, there are 7 players with 3 units each for a total of 21 units, the value per unit is (1/21)*700 = 33
Then a player leaves. The pot is still 700 in size, there are 6 players with 3 units each for a total of 18 units, the value per unit is (1/18)*700 = 39
All the players' units have increased in value just as if the civil disorder player had suddenly been destroyed; that player forfeit his points and resigned.
Now another player takes over the civil disorder player, with the new scheme. The going rate for units, from above, is 39 per unit. The player wants to take over a civil disorder player with 3 units. The required bet to take over the civil disorder is 3 * 39 = 117
Now there is a pot with 817 points in it, and once again there are 21 units on the board and 7 players. The value per unit is (1/21)*817 = 39
So the value per unit only changed when the civil disorder player quit, the value didn't change when the new player entered.
In the old way, however, players suddenly lose value per unit when a civil disorder is taken over; because the bet required to take over a civil disorder is too small so the player who is taking over pays less for the units than the units are worth.