Maybe not offensive. But definitely distracting and defeats the purpose of Twitter - which, for me at least, is to show me stuff I'm interested in.
Plus, it takes up a load of pixels on a device that is already short on screen space (a problem I have with all ad-supported mobile apps - I was quite happy to pay for Tweetie and I was quite happy to pay for Twitteriffic - and I am returning to using Twitteriffic full time).
He's raising a stink about a company's (poor) attempt at making money by causing its users the slightest inconvenience. That ad uses what, less than 10% of the screen space? Now, you can criticise how bad or ineffective the ads are. But, no, the dude is offended! He feels entitled to get even more from Twitter than what he's paying for, which is $0.
It's like if I got offended for Instapaper charging money for a premium subscription. No, I'm just grateful Marco's built an amazingly useful tool and is letting me use it for free. I'm not entitled to more than that.
The self-righteous sense of entitlement of people using free stuff on the internet never ceases to amaze me.