As @DSM pointed out, the powerlaw module deals with fitting an exponent to values drawn/generated from a power law distribution, rather than fitting a regression. To help people who might have similar confusions, below is how one should verify the exponent fitting:
## use a proper power law random number generator (or code your own)
from networkx.utils import powerlaw_sequence
pl_sequence = powerlaw_sequence(1000,exponent=2.5)
fitted_pl = powerlaw.Fit(pl_sequence)
fitted_pl.alpha
Out[73]: 2.4709012785346314 ##close enough