The reasoning of SABAM in this matter follows their tax on radio-broadcasters. Radio stations are obligated to pay SABAM for broadcasting copyrighted songs. So to rationalize their greed, they just followed that logic for internet providers. Since internet providers are now broadcasting a lot of copyrighted material, they should pay for it, just as well as the radio stations are.
It's just a bit sad that at the same time, there's a very steep price for internet streaming radio websites(to the point that you need to have big pockets to set up a legal streaming radio station in most of northern europe - even if you only broadcast non-copyrighted songs or songs by artists who aren't registered with SABAM). So not only are those website-operators paying, now also the ISP's should pay.
It's all getting totally ridiculous and there have been a lot of calls for a better alternative in Belgium. The current SABAM has more in common with the Italian mob than with a legitimate company. And on that note: Does is seem right the one who looks out for the rights of copyright holders, is a company? Shouldn't it be a non-profit?