+R was introduced later and they made provisions to allow bitsetting. Its in the official specifications of the +R standard.
The reason they allowed this and made it possible is the fact that [B]they arrived late to the market[/B]! They wanted to maximize compatibility for their media. This all became less and less important over time, as the vast majority of players could handle either format. These days it is hardly relevant, and I don’t bother with bitsetting any more.