Even if the expiry was extended to a year from the conversion - which it sounds like they were - I'd say you get a pretty good deal rather than ripped off. The credit life was extended beyond what it was originally. Just because the expiry date isn't listed on that page doesn't mean that there isn't one. And it actually is not all that hard to find.
If you check out the "Help" section on jetblue.com there's
this FAQ about the Travel Bank that is pretty clear about the one year expiry. Between that and the ability to search the transaction date for when the credits were added it shouldn't be too hard to figure out when they expire. Or you can call in and ask and they'll tell you.
Knowing that they used to expire after one year and not being told otherwise I'm not sure why you chose to believe that they existed in perpetuity.