They don't. Now the bonds are fully collateralised they can trade out anytime. Some people have an appetite for long term bond positions and would be willing to buy them.
Alternatively, again because the bonds are fully collateralised many banks will be quite happy to repo them for cash loan positions - so you've got your money now, can invest it as you wish, and then buy back the bonds back at point of maturity. Of course the REPO fees will be a minor minor hit, but that's just money going from one bank to another.
At whatever point Greece fails it's the eu taxpayers in it for the whole shebang (in terms of this re-writing of these bonds).