" I know it's legal for them to do this,"
And it's legal to dress up as a Klansman and walk through Harlem or Compton:-)
That said, I'm also of the opinion that IN THE EVENT of an accident caused when a car switches lanes at the moment that a splitting-lanes biker is there, it should automatically be assumed that the biker was driving too fast for safety. The biker knows the risks he's taking by splitting lanes, and it puts far more onus on the auto drivers to keep an eye peeled than it does on the bikers to assume cautiously safe speeds. If bikers wants to keep the privelege, they should be willing to cede legal right-of-way, so to speak, to cars changing lanes, and it should be reflected in deciding who's at fault come insurance-bickering time.