A reader adds more detail. In fact, Eagleton may not have given a press conference until July 25, but he was interviewed on television on July 14, the day of his selection, and on July 16, on Face The Nation. So Eagleton was available to some press one day after he was announced. From the NYT archive:
Sen McGovern pushes through nomination of Sen Eagleton as his running mate
July 14th Eagleton's comments on his nomination, made during CBS-TV int,
Sen Eagleton, appearing on July 16 on Face the Nation,
Sen Eagleton announces during July 21 interview in Washington (DC) that he will emphasize law and order and urban problems as the Dem Vice Presidential candidate this fall
Notice that most campaigns actually believe that it is good for them to get press interest in their vice-presidential pick. Normally, they can't wait to get him or her in front of the cameras. It's important to realize that, whatever the intimidation from the GOP, what is happening with Palin is without historical precedent.
The question we have every right to ask is: why?