I don't know any third party services which do this but internally we use that general technique to monitor a lot of conditions which overall I call "bitcoin network health".   As for why is nobody offering a double spend detection service?  Well one reason is double spends are incredibly rare, the fear far outweighs the reality.  The other reason is most merchants wait for at least one confirmation so they don't need to look for 0-confirm doubles.  It is important to understand that this type of network monitoring can't detect a double spend attempt when the double spend is not transmitted over the bitcoin network.  If an attacker has a deal with a "bad" miner to include his double spends in blocks for a share of the profit that will be undetectable until the block is published so even if you monitor the network you could be double spent.

0-confirm is not suitable for all businesses and one shouldn't naively assume they are safe but the reality is NO payment system is immune to fraud.  CC, checks, ACH, even good ole cash can be fraudulent.  Bitcoin doesn't need to be achieve a zero fraud rate, it just needs to be better than the alternatives.  For low value in person transactions 0-confirm is probably sufficient.  For donations, and digital goods which can be revoked (think steam games) 0-confirm works equally fine, for any product that is a subscription 0-confirm also works.  If you are selling gold bullion by the metric ton you probably should be waiting for confirmations (even 6 might not be enough security).