Several ways.... first, DVDs are made to exact specifications- adding a piece of paper will unbalance the disc and can be the source of erratic playback. Next, over time, it is quite likely the the label may bubble or peel in different humidity/seasons/age... leading to again an imbalance issue affecting playback. Finally, the deepest "pit" on a disc is closer to the top surface than the bottom surface, adding an adhesive to the top means that over time that adhesive could corrupt the disc. Although that may be avoided using an inkjet disc, I'm not sure.
Bottom line, sooner or later an adhesive label will unbalance the disc making it unplayable.
Mike