DVD+R, and -R are different hardware technologies, not all DVD writers are compatible, although more are coming (DVD-RW, and +RW are also distinctive technologies requiring specific hardware compatibility)
All 'high end' Macs come with DVD Writers, Apple completely supports DVD-R only, some Macs come with DVD Writers that also support DVD+R, but the best use of that hardware is with 3rd party software, such as Toast 6.
DVD-R is compatible with the largest percentage of stand-alone DVD players, DVD+R is somewhat less compatible, although there seems to be more support from the PC side for DVD+R .
there's not really a simple way to describe why the drive manufacturers sat on 4 different, incompatible formats for DVD-Writers (Apple is certainly not alone in this)