Thats because "Cross runway xxx" is a clearance, and is a clearance everywhere in the world.
Cross xxx at 6000 is only a clearance in the USA. It is NOT a clearance anywhere else. Everywhere else it is a restriction. The clearance comes seperately.
To cross a runway, you are instructed to cross. To climb or descend, you should be instructed to climb or descend.
Why is it so hard to be standard? "When ready, descend to 9000, requirement to cross XXXXX at 9000". Its not that hard.
But just like cross runway 7 is
explicitly understood as a clearance; "cross a fix at an altitude" can
be too too. Climb, descend, turn and cross
are all action verbs. There's not reason they can't be understood as clearances. You just aren't used to it. That doesn't make it wrong.
There are feeder sectors in the busy centers in the US which probably issue 40+ crossing restrictions an hour during the pushes. To take "Cross MINIUM at 9000" and turn it into "when ready, descend to 9000, requirement to cross XXXXX at 9000" is a huge increase in frequency congestion.
I don't believe that just because a procedure is done a certain way in the US that it's automatically better. By the same token because it's done one way in the rest of the world that doesn't make it better either. Just because you aren't use to something that doesn't make it bad.