WebmasterWorld administrator, Tedster, confirmed this to be the case:
I just went to the aol.co.uk webite and did "UK only" searches such as "keyword inurl:.com" -- you're right, AOL is choosing to show only .co.uk and .org.uk for their "UK only" searches. Not so Google.co.uk, where any domain extension can be found.
He adds that when doing a site command for .net TLDs, only a few come up. For example, a site:.net returns for me only 204 results.
This is probably a tweak that AOL set for this certain search criteria. Google powers AOL Search, but AOL is allowed to make changes to the results - if they see fit. They must feel the TLD is more important than the address on the site or the location of the server.
Forum discussion at WebmasterWorld.