Because using that ability doesn't take extra resting time? And because the bard's Charisma bonus is likely to be higher than the statistical average of a d6?
Except that it's not.
1) The PHB itself explicitly calls this out as a valid example of using encounter powers between encounters, as Smeelbo said.
2) This question has been asked roughly a thousand times since 4E came out. Customer Service answers and posts by the development team have all said that not only is the ability to use healing word outside combat within the realm of the rules, it was anticipated and expected by the design team.