Not allowing combination or transfer of certificates is not cheating the customer - failing to apprise customers of the restriction is cheating.

Doing a fairly rudimentary legal fraud analysis, the question is - is the concealed information material, is it something that the recipient of the certificate would like to know before agreeing to the transaction?

I thin the answer is clearly yes - if a passenger is told that in exchange for giving up a seat they will receive a $200 voucher for use on Delta, I think it is reasonable for that person to assume that the $200 is essentially like cash.

Moreover, it would be a very minimal burden on Delta to announce at the time volunteers are sought that the certificates are not transferable or combinable and are only good for one year. Also a minimal burden to Delta to mention that on the website when volunteers are sought. However, the result of concealing this information is that the offered vouchers seem more attractive than they really are, and more people are willing to accept them (and at a lower face value) than if Delta had made the minimal disclosure of the restrictive conditions.