Whats going on

Please tell me how if I sign a 2 year contract @ R99 per month how is it legal for Vodacom to increase the tarrif but I am not allowed to cancel my contract? What gives you the right to change things from the contract that I agreed to and signed? <br> <br> I recently upgraded and took a Vodacom Smart tab @ R99 per month with 2 Gigs free. I received a sms stating that that would now increase to R109 per month. I was upset with that but then today I get a sms stating my Smart S would be increased from R199 to R209?<br> <br> What on earth is going on?