Just be cause it's new, doesn't mean the battery is good/charged, etc.
Batteries, lose there charge a lot of ways, and most of them come from not putting a "FULL" charge on them, like a batt tender.
In your case, my guess is when you bought it the batt. was sitting in the car not being used, but as long as the dealer_peeps that move the car around before it's sold, can start it, makes it highly unlikely they're going to go to the trouble to put it on a tender till it reaches "100%"...... By the way, cars only keep batteries at 80% on a good day.........
I'd advise to buy a tender(a good one), and give it a 100% bust once a month, if you don't have inside parking, they make dash board solar tenders, just plug into interior power out-let and it will do it's thing(and they work on cloudy days just slower).
Dealer support: There are dealers that do the job correctly, I've seen lots where they'll go though the lot with a bunch/few tenders and bring the batt's back up.
"A lot" of people think that the car will bring the batt up to 100%, and it's my opinion they all should buy a tender. The more times the batt is kept at a high level, the least amout of sulfuric build up.
http://batteryuniversity.com/learn/article/what_causes_car_batteries_to_fail
http://www.walmart.com/ip/Battery-T...fault&beacon_version=1.0.1&findingMethod=p13n