Why do some spell checker mark "copiable" as an error?

Some of my spell checkers (including the one in vim, and the build-in one in gmail) mark the form "copiable" as an error, and insist on "copyable". This doesn't seem to make sense; the rule I always learned was that "y become i if preceded by a consonant when adding a suffix, unless the suffix starts with an i: eg "copied" or "copier", but "copying". According to this rule, the correct spelling would be "copiable", which is what I always use. But I often see "copyable", which doesn't seem right to me. (For what it's worth: Merriam Webster says that the word doesn't exist, in either form, the American Heritage Dictionary gives both forms, and the Oxford English Dictionary only gives "copiable".)

What is the correct usage, and why?

The 'rule' you've learnt doesn't apply to all words. There are more "exceptions to rules" than there are "rules". For example, you can't write plaiable instead of playable.
Some other exceptions are buyable, repayable, displayable, employable, enjoyable etc.

In case of copyable/copiable, they're both used, I don't know why.

According to Google Ngram Viewer, copyable is much more prevalent:

Both are correct and acceptable. Use whichever you want but be consistent.