Why Does the Update Statement also updates indexes?
For consistency. There are times the access path will use only the data from the indexes or will start from the index and jump to the table. In both cases the informatoin on these two entities should be compatible.

A index is just a separate ordered set of data from a table. When you remove an entry from the table, the entry on the index pointing to this row should also be removed. The same should happen when you update/insert. That's the burden of indexes: faster select and slower DML (insert, update, delete).

When you do, for example, a 

select count(*) from table t1 where c1 = :value;


Your result should be the same whether you have indexes or not. And the beauty of Database Management Systems is that they do that automatically for you. :)

You cannot prevent this update. The only time this will not happen is when you do not change the data referenced on the index.
If you have a table with four columns (a, b, c, d), and two of them are indexed (a, b), updating only the other two (c, d) will not trigger this extra update.