This does not seem correct. Here is a short non-comprehensive list of services by US companies not blocked in Iran. The list is big enough, and the companies involved large and legally-savvy enough, that this cannot be an oversight.
* Gmail, Yahoo Mail, Outlook
* Google Maps, Waze (quite popular right now in Iran too!)
* Google Drive, Dropbox, OneDrive
* Github
* Google Keep
* Google Docs, Zoho
* ...
The complete list would be hundreds or thousands of pages long. Based on this, I don't think your assertion is correct. Either that, or every big US company is breaking sanctions on Iran with impunity. That does not seem plausible.