Obviously not actually unlimited
Reminds me slightly of insurance companies. Make too many, high-value claims and they either refuse to insure you or raise the premium to such extortionate levels that the poor old customer can't actually afford it. All works well for the company all the while they're taking your money but if they have to pay out, it's a different story. So-called "unlimited" data deals were always too good to be true. ISP's don't like customers actually using the service as advertised. Throwing people off seems very underhand to me, if they are merely taking up what the ISP offers. A deal's a deal in my book and this does smack of welching but I guess there may be something hidden in terms and conditions that cover this.