the contributor is giving the company/project something. and not making a promise that they don't keep.

if i understand it correctly, consideration is only relevant for a contract that has not yet been executed.

once the contribution is made, there is no enforcement necessary, because the contract is already completed. the project must now be able to assume that it can use the contribution under the terms that were agreed upon.

if that was not the case, no project could ever accept any contributions from anyone.

all the CLA is doing is requiring the contribution to be made under specific conditions.

if the project only accepts contributions under the GPL then there is an implicit CLA that all contributions be released under the GPL. once the contribution is released the license for it can't be changed either.