The Dark Powers are helping Kas in his plan. He's a dark lord after all, and they explicitly allow him to go on probation to face his rival. They TOLD him about Venca's plan! So when the Ro7P part shows up in Barovia, the Dark Powers allow the PCs easy entrance and egress because it is furthering Kas's plan. The PCs don't know that, of course.
Speculation: why would the Dark Powers allow Kas out in the first place? Because they love to torment those they hold, and letting Kas go to torment Venca (who is the only dark lord to ever escape) and then, once Vecna is defeated, to snatch Kas back before he had a chance to remake reality, sounds like a really great twofer in the torment department. Of course, the PCs stop Kas sooner, but the fact he got so far into his plan and then failed to stop Vecna before being banished to the Mists is sufficient enough punishment for now.