...Because frankly it doesn't make sense, not when you break it down bit by bit. The A's have told Piazza that rather than getting ready to DH now, he should wait 2-3 weeks until he can rehab to be the backup catcher.
First of all, this means that for the next 3 weeks the A's would rather have him not hit for Oakland than hit for Oakland--a bit of a slap in the face for the best-hitting catcher of all time whose DH competition is a streaky career minor leaguer. Ostensibly, the A's reasoning is that Cust is raking again so the DH role is covered for now, and they might want to have both Cust and Piazza in the lineup in the second-half of the year. And that's where the A's are approaching it all wrong.
You see, there are two ways to get Cust and Piazza in the lineup together. One is the way the A's are exploring it (a way that stalls Piazza for 3 weeks as a hitter), and that is to have Cust DH and Piazza catching, while Dan Johnson plays first base. But there's another way to get Cust and Piazza in the lineup together, and that is to put Cust in LF, Piazza at DH, and Swisher at first base, with Kendall catching.
The "A's way" puts Johnson in the lineup instead of Kendall, and Piazza in the field instead of Cust. As a catcher, Piazza couldn't throw well last year, and his rehab is being stalled by the fact that currently he can't throw at all. If Piazza, whose right shoulder was an extreme liability before he injured it severely, plays behind the dish, is his defense going to be less costly than Cust's in LF? Both are awful, but not every ball is hit to LF--whereas every pitch is thrown to the catcher.
Meanwhile, "my way" trades Johnson for Kendall in the lineup, which trades a bad power hitter with a poor track record for a bad singles hitter with a good track record. It also puts Kendall's legendary game-calling/pitcher nurturing skills at catcher and upgrades the infield defense with Swisher, to help offset the sharp downgrade in LF.
"My way" also gets Piazza's bat back in Oakland now, while the "A's way" asks Piazza to store his bats for an extra 3 weeks to prepare for a gig which his right shoulder physically can't handle. It's as if the A's are telling Piazza they don't think he can hit well enough anymore to contribute now but they do think he can throw well enough to contribute later, and if that's the message I'd say the A's are wrong on both counts. Which is why I suspect that whatever he says publicly, Piazza is probably none too pleased with the latest turn of events.