The bitcoin blockchain is around 45 gb today. On a 2mbps connection it will take about 6 hours to download, right? Nope.
It takes much much longer, why? Because the validation of the correctness of transactions is a CPU intensive process.
In Bitcoin core, peers would not only reject Bob, but also penalize Bob for sending too many invalid blocks. Unless Alice is paying Bob more than 25 bitcoins for each such attempt, Bob is better off sending empty blocks rather than invalid blocks. In BitcoinXT, further Bob's identity also matters.
It is against the bitcoin protocol anyways : See point 16
https://en.bitcoin.it/wiki/Protocol_specification#blockHowever, many wallets do not implement the protocol correctly. For eg SPV Bitcoinj wallets do no validation what-so-ever, blindly following the longest chain.
There was this issue in July where miners were SPV mining, that is blindly mining over the previous blocks without validating and generating invalid blocks. Details:
https://bitcoin.org/en/alert/2015-07-04-spv-miningYou might also want to read the discussion around BIP66.
So yes, in bitcoin, no one trusts anyone and verifies everything for themselves.