Obviously SaaS providers will not offer kickbacks to startups, the deals aren't usually big enough. I've witnessed it in a big corporation once. One of the engineers was VERY insistent on using a specific solution even though it didn't make sense technically and everyone else was against it but because they were more senior, they made the final call. If they don't get outright bribes, they will get lucrative job offers from these big companies in the future.
Imagine being the guy who convinced Coinbase to use DataDog... That person will probably end up working at DataDog sooner or later if not already there... You can bet they will be getting a very cushy salary.
I could probably make a living out of extorting corrupt engineers. It's so predictable.