When you start a subscription, you're agreeing to pay X amount every Y period of time; you're not starting a new agreement every single Y period of time.

They can cancel the prior tier or bump up the price on renewal though. This is the problem with subscriptions, you become complacent and accept incremental changes until you finally notice that you’re being rinsed.

And actually some subscriptions can include unilateral price increases in the contract (a subscription is a contract) with early termination fees. It just isn’t commonly done because word gets around and you will lose business. You typically only see this in predatory industries where there are few alternatives and the service is necessary, like local waste management.

If the contract is unfair enough you can usually escape it in court or arbitration, but nobody wants to go through that.

No, that doesn't make sense at all. You've paid for consistent terms for that Y period of time. Not cancelling the subscription when it's up for renewal is an implicit agreement to any new terms. And I'm sure if you'd read those terms in the first place, you'd come to the same understanding.

(And it's not even that: the X you're charged is subject to change upon renewal!)

I'm not arguing that this is a good or bad thing, just pointing out the reality of every single subscription agreement I've signed up for online.

Of course you are. Either party can adjust a contract on renewal, just like a lease.

Also you aren't agreeing to pay to renew the contract. It isn't a rent payment in a structured contract. You can cancel at any time.

They can cancel the subscription if you don't agree to the new proposition after they fulfilled their contract. But they can't just change the terms of the agreement after it was made.

But doing so would mean risking to loose customers who were just too lazy to cancel. So most Businesses don't like it. (Spotify did cancel their old contracts though, for people who had not agreed with the recent price hike)