Do banks care about down % or equity %?

[Samuel Bavido]

I was recently looking at a listing for 35k that claimed the property was assessed at 75k. Now, I have no idea if those numbers are anywhere near correct, or where they came from, but it got me thinking; if the property was indeed worth 75k, then a 70% loan ratio should allow you to pull about 17k out of the property right away for rehabs. But I've never heard anyone discuss this strategy, which makes me think I'm probably missing something. If I were to find a deal for 50 cents on the dollar, what is to prevent me from borrowing 70 cents on the dollar, walking away with an additional 20 cents?