Ahh yeah that's a bit confusing, the merge topic tool can be used between categories, so you should click merge first and then select the topics. The placement seems like a theme/skin issue.
Allow admin to move a topic to a different category
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The is a key moderator functionality to relocate content that users post into a wrong category
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@Baerrus said:
The is a key moderator functionality to relocate content that users post into a wrong category
Thread Tools > Move Topic