This is very common with event branding. In fact, one can often be asked to make one sponsor logo larger, or more prominent, than another due to an increased contribution.
Be thankful you are only dealing with 2 logos, many cases can have a dozen or more logos which must be included.
The "valid" reason is the sponsorship itself. If a company is donating or otherwise contributing to an event in exchange for the advertising it generates... there's your reasoning. They want their logo shown so they benefit from the advertising.
The key, I've found, is to not design the event brand around any sponsor logos. But rather add the sponsor logos as an afterthought to a solid event brand. You should design the event brand as a solitary item without considering any sponsors.
In your two image samples, the one on the left is problematic due to the sponsor logo integration. Whereas the one on the right can have a well-designed event brand and then merely place sponsor logos beneath it. That way the in areas where sponsors are not required to be shown the brand is still a solid, appealing, and self-contained graphic (e.g. the front of t-shirts with sponsors on the back).