I have never met Sherri Poterfield so obviously I can only speculate on why she made this choice. But that I will do!
The Poterfield's melody begins with a musical structure that is called a "Satz" (or sentence). A Satz consists of two halves: the presentation and the continuation. The presentation has two halves: first comes an "idea", or a motif (musical rather than literary), then a (mostly varied) repetition, both parts are of equal length. The continuation is slightly hairier but in the textbook case the continuation is the same length as the presentation, yet doesn't break apart in two halves and, while based on it, does not literally repeat the full motif from the presentation.
A Satz is a very old, very reliable and very common thematic structure, especially for a beginning, and Poterfield decided to open the piece with one. This means, however, that she needs a melodic idea she can repeat. The idea of a Satz is generally very short, in this case 4 notes (which is not at all unusual, compare Beethoven's fifth symphony), since "tiger tiger" is 4 syllables. However, to repeat the idea she needs another four, while "burning bright" is only three. A (small scale) Satz is a very rhythmical structure, usually the variation of the repetition is a transposition or a melodic variation (and the Poterfield setting of The Tiger starts with an, apart from the text, unvaried repetition), not a rhythmical variation (especially not such a big one – omitting a quarter of the material). In addition, a common technique for the continuation is shortening the initial motif, so doing that in the presentation creates problems in this area too.
In conclusion, adding the extra syllable is very convenient (but by no means necessary) for the melody.