Not sure if they're purposefully blocking uBO. Those who are effected will either move to Firefox (and encourage their friends to), or just use another content-blocking technique (eg, DNS).
For the few extra ad clicks they'll gain, it's just not worth the upset.
Their captcha is deliberately vindictive against users who don't have google accounts, use firefox and use uBO. For instance, the tile "fade in" lasting several seconds that plays absolutely no role in making the captcha difficult for ML to solve, existing only to punish real humans who don't comply with Google's surveillance system.
So in light of that, I see no reason to not assume malice in this case too.
For the few extra ad clicks they'll gain, it's just not worth the upset.