SBST's style is much more open.. They did a whole body AD for
the buses with the SG50 and Majulah Singapore / Jubilee thingy.
For eg. SBS5011L (HGDEP 113)
I believe SBST's NDP ad this year isn't by SBST's own initiative
though, ie. I think the NDP committee awarded / bought the ad
contract for the NDP 2015 ad to SBST instead.
The official NDP ad hasn't been on SBST buses for quite a few years
already. IIRC the last NDP ad on SBST buses was way before 2010.
Past 4 yours (2011-2014) the NDP ad was on SMRT buses (Bendies
primarily). 
So this sticker by SMRT seems to be SMRT's own way of
commemorating the event this year instead of leaving themselves
out. haha.
I think some of the SBST buses will get the sticker too lah. But
considering the amount of buses with the 3D ball pop-ups &
Spot-Me stickers, SBST went on a larger scale already.
