Promotion contract price increase

Good day, I had taken a WiFi contract for 12 months with telkom while Telkom had a promotion running at R399 per month and a couple of months later there is a price increase on the contract. When I call to enquire, I am informed that Telkom had an increase across the board on their products and apparently they are allowed to increase a promotional contract. This does not make sense to me, if there is a promotion and a contract in place for that promotion how can there be a price increase? If I take a cell ohone contract or any other contract the fixed amount for that contract is the amount I pay through the contract period. This makes me doubt the itergrity of this company, are the promotions run as a ploy to get people to sign up and then increase the premiums to what they actually want, some may say this is entrapment. The increase in their premiums does not make them competitive in the current market but you are contractually obligated to them. The cherry on top is that the amount of the initial contract gets debited from my bank account and I now I have to pay the difference from the new premium manually or they will disconnect my service. This is very ********* behavior, one thing for sure, I will never recommend or use your services ever again.