It's not an unfair advantage so much as the team was just messing with one of NASCAR's three golden eggs...do NOT mess with the size of the engines, the fuel or the tires. These three rules have been set in stone for ages now, and these are the three areas guaranteed to get you on NASCAR's bad side if you mess with them.
From a technical standpoint, small changes in tire pressures can have surprisingly large effects on tire contact patch sizes, and changing the tire contact patch size a slight bit can produce large variations in a car's handling characteristics. Tire heat is the #1 enemy of handling, period. That's why NASCAR tires have sidewalls so thin that you can make one go flat just by walking up and kicking it. Less mass flexing in the sidewall means less heat buildup, and also less mass to retain heat and keep it from dissapating. If a team were to bleed off this extra pressure as it builds, then their handling characteristics could stay much more consistent throughout a run, which will produce a faster, more consistent car.
NASCAR has been monitoring what the teams are doing to bleed off these excess pressures as tire heat builds since the middle of last season, and there are various ways to do it. Some teams have been putting pin holes in their air valves and some have been leaving their air valves not fully tightened. Until now, though, NASCAR has not caught anyone actually physically altering the tire itself, and to date, we have not been issuing penalties for the other stuff that we've been seeing. It's pretty easy for us to see who is bleeding off pressures...when we have to pump up the tires just to unload a car off the NASCAR hauler every week, then it becomes pretty obvious.