The throttle value you're mentioning is specific to cron jobs, not to a single send event. There's no easy way to throttle the number of mails delivered in a single send event.
In general, the main reason someone would want to use cron in this case is when your subscriber base is large enough that a single "Send" action will time out, due to the time taken to send the total number of emails exceeding your server's PHP maximum execution time directive.
Cron is utilised in this case to break up large subscriber bases into chunks your server can better manage.
In your case, you have a limitation of 20 outgoing mails imposed on you by your service provider. To ensure you can send mail to more than 20 people at a time, you will have to use cron to get around this limitation.
The throttle you refer to is the number of messages sent PER cron. Removing cron isn't advisable for a substantial list, as your server will connect to Amazon, attempt to deliver all the mails, of which only 20 would be accepted.
Without knowing the inner workings of Simplenews, one of two possible (and undesirable) scenarios would unfold:
1.) Any mail in excess of the first 20 would be ignored by Amazon, and sending would fail. Your site would ignore the errors, and of your entire "Send" event, 20 mails would go out.
2.) Simplenews might cache the remaining mails that Amazon doesn't accept in your DB. These would wait around to be sent at a later stage (next cron run), but because you're not running cron, Simplenews wouldn't be invoked during cron, and that queue would never be properly addressed. This would result in you sitting with a large number of unsent mails clogging up the Simplenews spool table.
Bottom line, disable cron at your peril. The second your subscriber base goes over 20, your newsletter sending process will start to give you issues.