I agree, people could down-sample, people could change tags, people could do a lot of things. But most of them won't. Simple as that.
How apple will check, and how users would work around those checks if they did happen is all speculation at this point. I guess we'll see as the new feature is rolled out.
I'd argue that making sure that people don't use iTunes Match to match stolen files would be a lot more important that album art, but I guess people prioritize different.
If they can't get a good hit rate on something as simple as album art on files that are tagged properly, it's not a good sign that they'll necessarily do much better when they have to add the challenge of examining the content itself. The priority isn't getting album art right, it's getting matches of legal files right. And that definitely seems to be a problem so far with a very small percentage of files getting matched properly. In theory it may not be hard to reject tracks that match a blacklist, but at this point they don't seem to be doing well at recognizing legal non-iTunes tracks. And that should be their priority since the service will bomb if they can't get that right.
I've already paid Apple $10 for an album. They should want to deliver the music to my device.
For songs you bought from apple, they do deliver that song again to any device you own. The feature exists already, it appeared weeks ago. This service is only needed to deliver songs you did not buy from apple. Not to mention that any songs lower than 256 bitrate are upgraded to that - I'm sure there will be plenty of people who will get it for at least one year just for that (and once they have it, I'll bet most stick with it).