Other search providers do not have a monetary partnership with Opera so read into that what you like.
There's ZERO EVIDENCE the contract between the search engines that partner with Opera says they'll be the only ones able to be set as default.
"The option to set a custom search engine as default was (temporarily) removed to combat malware overriding user settings and directing all searches to malicious third-party sites. Its an unfortunate situation." - Opera employee Daniel Aleksandersen.