Don't think this is the reason, at least for Sentry. They've been in business for more than a decade.
The bigger reason is that it would take AWS 2 weeks to offer a version of their product. It would be API compatible because it would be exactly the same code. AWS would offer an introductory price of $0 for the first year. Not all Sentry customers would switch, but many would. Sentry would continue to develop their product, while AWS maintained a skeleton crew to deploy their changes and make most of the money off it.
While it may be easier to assume the worst of people, ascribing all kinds of intentions to them, just listen to them. They say "we don't want AWS to deploy our product and take all our customers". They say it because it's what they're really worried about.