The most likely reason the IReadOnlyXxx interfaces where added in 4.5 was because they were required to make .NET collections usable in WinRT projects (Store and Phone apps). Necessary to properly map the collection to WinRT's IVectorView<> and IMapView<> interfaces. This is done automagically by the language projection built into the CLR. With the clincher that WinRT doesn't have the equivalent of an ISet<> interface so there just wasn't any need to change HashSet<>.
Update: the asymmetry was resolved in .NET 4.5.1, no doubt thanks to plenty of customer feedback :), HashSet<> now also implements IReadOnlyCollection<>