Perhaps the Mets didn't know, but sometimes you'll have to battle different playing conditions while on the road, especially if they're facing a rival who's battling for a playoff berth.
The New York Times had an article today in regards to Jose Reyes and the Mets complaining to Major League Baseball because of a wet infield, especially around first base. They made the accusation the Braves drenched the infield because they couldn't stop Reyes, Angel Pagan, and other Mets on the base paths and watered the infield down in hopes to prevent multiple stolen bases for New York.
I guess they're called the LOLMets for a reason.
For one, perhaps the Mets didn't realize this, but it was hot, humid, and sunny yesterday evening in Atlanta. With the temperatures being in the low-90's and the sun still setting, any wet dirt will dry naturally within minutes. If they don't water it down the players run the risk of kicking up dirt and tripping as they round a base. It would be no more of an injury risk than over-watering the infield is.
Getting any advantage on the opposition within the rule book is a big part of baseball history. If your team's pitcher induces a lot of ground balls, the grounds crew doesn't cut the grass. If the opposing team's pitcher gets a lot of grounders, you cut the infield grass just hours before the game starts. If the opposing pitcher is a fly ball pitcher, maybe an outfield light or two magically don't turn on that night. It's not cheating. It's a home field advantage. If you don't take advantage of your home ball park, why even bother playing home games?
This is the kind of stuff that made Braves fans hate the Mets so much over the last two decades.