Good point. Yes - that is a problem if the ropes are not anchored down properly....or if the TD does not declare the ropes a guide and straight lines between the anchor stake the offical line.
Is this a lot different than the level of a stream varying during the duration of a tournament (if it is not staked off for an official OB line somewhere on the bank?)
Yeah but the ropes aren't put down in exactly the same place every year either. The stream is always there, sometimes it's dry, sometimes it's flooded but it's still always there. You're not going to show up to the tournament ready to make up for you mistakes last year only to find out that they moved that stream you OB'ed on 10' closer to the hole or something. There's something a lot more appealing in learning a course and adapting rather than having it change on you drastically every year. Playing a course with a lot of artificial OB is like being in the X-men's danger room, novel but hard to get nostalgic or sentimental about it. :\