Intentionally chose an open ecosystem for the smart relays in my house, now they crippled the app adding a $4 subscription
If you don't pay a subscription you can't see the history of use, it's limited to the latest 5 events. When I bought the ten devices (=I paid 150 euro for the hardware), it was 100 events
Whether or not the price is right is another discussion, but I will point out that they're not charging for you to see the events. Like you said, there's a web server and API for that. They're charging you for them to store the history and serve it up on demand, which does have ongoing costs and I understand not accepting a one-time purchase for.
If you want history, hook into the API with Home Assistant or Grafana or something and store it yourself. It's either that or pay them to store it. Infrastructure that stores and serves up the history doesn't come from nowhere, and an ongoing cost for an ongoing service kinda makes sense.
I think 100 dollars is more than enough money to cover a few bytes of text storage. It's not like they are storing megabytes of tracking meta data. And if they are that's even more reason to provide the service for free.