Whether or not an Xbox One game has a demo is entirely up to the developer of that game — even if it's downloadable, said Microsoft's Major Nelson yesterday.
In response to a comment asking why there was no demo available alongside yesterday's release of Peggle 2 on the Xbox Games Store — and specifically, why it appeared that Microsoft had changed its demo policy in the transition from Xbox 360 to Xbox One — Major Nelson said, "It is (and always has been) up to game developers to do demos for their own games. They'll be more demos for Xbox One as the platform goes forward."
On Xbox 360, Microsoft requires all Xbox Live Arcade games — the smaller experiences usually priced between $5 and $20 — to have free demos available to users. However, on Xbox One, there is no distinction between smaller games and full-size games; there's no such thing as XBLA on Microsoft's new console, since most of its full retail games will be available for download in the Xbox Games Store along smaller titles such as Peggle 2.