Yes, UVI Workstation is completely free; there are no "catches" per se.
This seems to be a common tactic with companies that sell sampled sound sets-- they provide their sample player free of charge, often with a free sound set containing a relatively small number of sounds.
Then they have more sound sets for sale which require the player.
That way they can get you using their sample player and whet your appetite, and when you're hungry for more you can buy the specific sound sets you're interested in.
UVI has a number of sound sets for sale-- some are orchestral instruments, others are pianos, or organs, or synths, etc.
I have a few UVI sound sets; the first one I bought was a special synth collection that was supposedly produced for Guitar Center in honor of their 50th anniversary, and (if I remember correctly) the deal was that if you bought another product you would get the synth collection free, so I bit.
Then I bought a few of the less-expensive sound sets when they went on sale.