Ads on the lockscreen may be fine from a UX perspective, however, that tablet is now using data when it's a sleep which becomes a factor when someone is on a capped connection. Sure, it may only be a handful of MB/month, but to some people, every MB counts.
I believe this thing only has wifi, so that's not a relevant problem.
This might come as news, but wifi data is still data and counts towards the data cap.
Its not some free magical data flying around on its own.
In what third-world country do you have to live for your home DSL (or fiber or whatever) to have a data cap? Even back in the olden days of dial-up that would be laughable on the face of it.
Can you turn off the lockscreen?
Don't think 'lock' think 'swipe over to make sure this isn't a pocket dial' screen.