1day unlimited data which has limits

I purchased mtn 1 day unlimited data for 40 rand which was a waste of money,little did I know that once I've used 150megs my internet speed will be cut down drastically mtn says it will be 100 and some kilobytes per second but for me it was averaging at 9 kilobytes per second on my LTE tablet(and yes my lte service is on), my question is why cut the speed in the first place am I paying 40 bucks for 150 megs of good data only,because quite honestly I had to wait 30 minutes to download a 8 megabyte video.if I wasted 40bucks on 150 megs of good data speed why didnt I purchase mtn daily bundles or weekly bundles which when I last checked I could buy 100 megs for 15 bucks, mtn is just ripping us off with this 1day unlimited data especially when the speed drops after using just 150 megs of data,now when I think about it all the time that I have wasted in downloading 5 files which was less than 400 megs combined I could have had a short nap and I could have woken up just before midnight and taken advantage of the 1gig night data for 10 bucks on the same mtn network, that 1day unlimited data for 40 rand is a waste of money and time