Wrong sub-forum and duplicate subject anyway.
Google didn't take any choice away. Yahoo!, Bing, Yandex and Baidu can be easily set as default while there are workarounds for others.
"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.
"This will be renabled in a future update. Currently disabled due to third-party applications overriding the user’s preference in favour of malicious sites." - Opera employee Daniel Aleksandersen