Try cleaning with warm soapy water first.

If the epoxy was not mixed correctly and failed to harden fully the tables are junk.

You can buff out scratches, but it take a while and VERY fine abrasives.
A kit for removing scratches from plastic will have the correct grits.

The epoxy coating I use, EX-88, can be reapplied to a cleaned and sanded existing coating. After 12 years, the stickiness may just be built up grime and cleaner residue. Epoxy mixed improperly will never harden, like brickeye mentioned, but I doubt that's what happened (since they have been in service 12 years) You know the next day if you did the epoxy wrong, and yes, the surface is basically a lost cause.

I would try the EX-88 over the existing epoxy, after it's been sanded and cleaned thoroughly. Test one small table and see what happens. Any chance you can find out the name of the product they used originally? That would really help with the compatability question.

You can repair the finish by sanding and buffing the epoxy, like brickeye said. I start off using the random orbital sander w/ 220. This removes the damaged surface and grime. Epoxy finishes are really thick, 1/8" or more is typical, so don't worry about sanding through to the wood. Once the damaged layer is sanded off, I follow up with a rubber sanding block, wet sanding with 220 and work up to 1500 grit. Wet sanding is key with the epoxy to keep the paper from gumming up. Use regular water with a tiny drop of soap. I use a bucket and dunk the sanding block frequently, cleaning it underwater with my hand. After the 1500 wet sanding, wipe it down with clean rags, and let dry. Now buff it with 0000 steel wool until you get the desired sheen.