Interesting it's not something I've noticed before. I'm 99% we don't have these in the UK (although on country roads there will sometimes be a sign indicating a sharp bend).
The problem of speed limit overuse is small or non-existent in western Europe, but common in central and eastern Europe - it's even recognized by our officials as a huge problem causing crashes.
Not only there are speed limits before sharp turns, but unnecessary small limits in regular turns or on straight motorways, which makes people go over the limit even when the limit makes sense.
In many European countries, there actually is a speed limit before almost every sharp turn that's in or near a bigger city, and thus people begin to think like this.