It has nothing to do with home and away. That only applies to the NFL. TBS and WGN are two different animals. TBS' baseball contract is considered a national broadcast contract not specific to any one team. Therefore their games are blacked out in the local markets (just like ESPN on Monday night.) WGN's deal with the Cubs is no different than the D-backs had with KTVK up until this year. It is simply a local broadcast that happens to be on a superstation. Had KTVK been a superstation carried in Chicago last year, viewers there could have seen both feeds as well.
But all of this is likely to change in the near future with the Cubs being sold and Tribune set to "rebrand" the Superstation feed of WGN. Soon enough Cubs fans, just like all other baseball fans, will have to get the baseball package in order to watch most of their team's games. Maybe then the D-backs can pick up a few more fans???