is/2 is not Datalog though- it takes an arithmetic function as an argument; +/2 in your example. So the above would not terminate because it's Prolog, not because it's a Datalog program evaluated by Prolog.

>> Datalog doesn't even allow terms as predicate arguments so of course you can't construct them of infinite size.

Usual confusion: I think you mean "terms" as in Prolog terms, i.e. atoms of predicates (as opposed to terms in logic programming, i.e. variables, constants and functions). Datalog accepts constants and variables so with a finite vocabulary you can't create infinite atoms. I don't think that has to do with bottom-up or top-down evaluation.

To be honest, I haven't read any Datalog papers, but I'd be surprised if the termination guaranteed rested upon a specific implementation rather than the language semantics. Maybe not surprised- but it would be less interersting if you can only guarantee termination if you implement the language just so, vs. if it's a property of the language.