The allow filtering command is there because your query may result in a large (or at least undetermined) amount of data being read that will not be used in your query.
select value from eventindex WHERE column1 > 1234567890 AND column1 <= 1234567895;
This query will need to read every row - as there is no way to know if a particular row has column1 values in that range without looking at the row. Rows that don't satisfy the WHERE predicate are then filtered out.
Considering how many rows there may be in a large data store, this is understandably not allowed by default. However, in some cases you may know enough about your data to say "this is reasonable, and I want to do it". In that case, you can allow it by using ALLOW FILTERING.
When you use a predicate that specifies the partition key value (e.g. WHERE key = .....), then you wont' get this message, because only the row(s) that satisfy this part of the predicate need to be looked at.
See the following for more details:
http://cassandra.apache.org/doc/cql3/CQL.html#selectStmt