It's presumably laid out in the contract that their users sign. IANAL but I understand contract law gives wide latitude for parties to make agreements to do things which are generally legal.
One could imagine a legal regime that prohibited price or service changes in otherwise-legal contracts. Of course, that also might not be ideal. For example: a vendor used by the provider suddenly changes the capabilities of a piece of equipment such that the original service sold by the provider can no longer be provided at any price. What everybody wants in that scenario is for you to be seamlessly moved to the closest reasonable alternative, not for your service to automatically terminate. Similarly, most people will gripe about a $10/mo change in price, but against the likely alternative of automatic cancellation, the increased fee is what _most_ people will prefer.
And again, 100% of them will have agreed to this exception-handling clause when they signed up.
> This is like if I order a $200 SSD from Amazon and instead they ship me a $300 SSD and charge me $300, and there are no returns if I don't cancel the order before it's fulfilled.
There is a material difference in changing prices (say) 5 years after a contract was signed and ordering a specific item at a specific price a week or two earlier. IANAL but I believe the ordering of goods via the Internet is in fact covered by different laws in a way that services are not. Which again, makes some sense given that service contracts frequently have a term of "forever," during which time everything about the world can change. (Pretty sure my current phone contract was signed over 10 years ago. I'm glad they changed my plan to let me use LTE, for example.)