I can't imagine it is really a big water treatment issue since they have a different density than water and you could separate them with settling tanks and skimmers.
Separating really small objects of almost the same density as water (0.91â"0.96 g/cm3 - they are made from polyethylene) is not easy, and the fact is that they pass through all existing water treatment works. Plastics are in fact a serious environmental issue, 1) since they often leak hormone-like chemicals, and 2) because plastic objects are mostly not broken down into their chemical constituents, but instead break up to form very small plastic splinters and fibres. These are now found everywhere in our food chain; certainly in anything that starts life at sea: fish etc. We still don't quite know what harm they cause - the great worry is that thei will turn out to be as harmful as asbestos. Is it a good idea to allow the industry to pump these largely unnecessary products out, when it seems likely that it will cause massive problems for society down the line? Health problems cost society money, not just in form of hospitals, doctors etc, but also in lost productivity - prevention is better than cure, and it is also better for business in the long run.
And I don't see it matters for industry really because they'll just go back to using what they were using before which is mostly - sand.
You use this stuff as an abrasive and maybe the microbeads are mildly less abrasive? I don't know... anyway, they'll just replace this with very fine sand.
Sand is a natural material, and the environment already knows how to deal with it. I don't know exactly why they prefer to use plastic, but I'll bet it has to do with thei short term profit. Maybe it is a selling point, or was - I remember when it was first introduced and you suddenly heard a lot about how harsh the old kind of toothpaste was to your teeth. In reality it is probably no more than a selling point, like the current craze for putting triclosan in everything - which doesn't actually kill bacteria, but is likely to harm our health in the long run (both directly and by breeding resistent bacteria; when will we bother to learn?)