[plt-scheme] likely 3.99.0.22 -> 3.99.0.23 incompatibility: hashtables

Matthew Flatt wrote:
> Immutable hash-table update will be constant time. (Well, O(lg n), but
> the constant will be so small that it's effectively O(n).) Mutable
> hash-table update would be O(n).
>
> That's different from lists, where operating on the two kinds have the
> same complexity, and I think it's a sever enough difference to
> discourage the operation on mutable hash tables.
>   
Why O(n) if you already have O(log n) immutable hash table? Isn't it 
very easy to define at least equally efficient hash table then? Is it 
political or technical decision?