If not, why are there ".onion" URLs instead of just ".com" URLs? Is it simply to hide the web-service from non Tor users?
If you use a public domain name registrar, they may choose to route your web site to a third party, if for example, they are forced to do so by someone (under duress, court order, law enforcement request, or some less legitimate reason)
With an .onion domain nobody can seize it, as there is no way to impersonate your private key. There is no registrar to corrupt or force.
why are there ".onion" URLs [...] Is it simply to hide the web-service from non Tor users?
Anyone can be a Tor user, so hiding from non Tor users won't work.
It is mainly to hide the location (IP address) of the server, as part of hiding the identity of the owner. Not just from non Tor users but from everyone. (And as Harry Seventyone wrote, .onion 'domains' can not be tampered with by registrars.)