Why full index scan over full table scan?
If the index contains both cid and email, there is no need to look at the records.  All the data will be retrieved from the index.  This is a common optimization. 

The where clause should (on average) reduce the record count to less than four percent of the records.  This should be a small enough set to make using the index faster than table scanning.  Different engines have a different cost cutover point. Matching the wildcard on the index is likely faster than than matching against the record (column position matters). 

In a case like this, the optimizer will be guessing data distribution.  The optimizer likely doesn't have stats on a trailing match, so it will likely be guessing based on 5 random characters.  This would severely tilt the query in favor of an index scan, as the expectation is that only a small (much less than 1) percentage of records will match.