I thought that the problem with the 51% is the allowing transaction confirmations rather than whether your pool receives a block award... but it has to do with being able to transact with money that you do NOT have to then steal and then to undermine confidence in the network b/c you control the approvals of the transactions.
The problem with an entity having more than 50% of the hashing power is that it basically can decide the winning chain.
Below 50% it can't go off and do its own thing as it needs others to contribute. With more than 50% it will always eventually make a chain that is longer than a competing one