I thought you were going to show a virus result - the screen you showed is very unambiguously an opt-in screen, it even has the term "Accept" (as opposed to some other word like next or install) making you look at it - if the text said something else you would be very likely to just click it. The button at the left 'skip' is also clear about your other option (if it said 'cancel' it would sound like you're aborting installation.)
I just don't see how this is a trick. There's not even a "default" option here, except for the Accept button being on the right. How can anyone possibly click the word "Accept" without even glancing at the text surrounding it - even a glance shows you the giant icon with the slogan "Mac Cleaner Clean Up Junk Files on Your Mac"? If you look at the bullet points, the first one clearly identifies what it does. So it's an opt-in screen. There's no default. The action word is "Accept" and the top bullet is about the additional software you would be installing, with a giant picture and caption showing the same thing. Or you can click "skip."
This is above-board if you're going to do this sort of thing. Hell, given how above-board it is I might just click "Accept" in this case! (knowing that the devs opted into it as well.)
It is very, very hard to read this as anything other than an opt-in screen. How can you be any less ambiguous??
EDIT: I didn't even notice, there is even grey small text that spells this out in completely unambiguous words! (Thank you for considering this offer from our advertiser...your choice does not affect installation of FileZilla.)
I honestly don't even know that I would ask FileZilla to change anything whatsoever about this screen. It's great!