There is consideration. It simply is intangible. You get use of the
source code, the author gets intangible rewards -- pleasure, spread of
personal fame, etc. I've asked lawyers about this sort of issue in
the past. According to them, it is unlikely that any modern court
would refuse to enforce a copyleft on the basis of a lack of exchange
of consideration -- this is (apparently) rarely if ever an issue in
any modern contract dispute, simply because "consideration" is such a
fuzzy concept.
I know fairly good lawyers (like Lance Rose) who claim that a properly
phrased "free software" license is indeed enforceable in court.
Perry