Why do US people refer to the toilet as the "bathroom" when in most cases the room in question does not have a bath in it?

Sorry Mark, but you're mistake here...In most private homes the bathroom does have a tub. In public facilities this is not the case, and they are usually called restrooms.

sorry but the terms can be confusing.
I had an American guest many years a go who asked me to show him to the bathroom, which I did. He actually wanted the toilet which was a different room altogether. A toilet is a toilet and a bath is a bath.

Here in the States, the word "toliet" is rude - and we generally try to avoid using it. To say bath room, or rest room, or facilities is much more polite then to say toliet...

Yes, but even so, it's not a bath or a rest you're going in there for, is it? How about 'lavatory', or is that just as bad as 'toilet'? I remember a sketch on a British comedy show, featuring the comic duo Ronnie Corbett and Ronnie Barker. Corbett, who was playing a guest at a cocktail party, was using every euphemism under the sun in an attempt to ask 'politely' to use his host's toilet. He said to Barker, "May I use your bathroom?" to which Barker replied "It's a strange time to have a bath, isn't it?". In the end Corbett's character was so frustrated at his fruitless attempts to make himself understood, he shouted across the room to all the other party guests "Yes! I want to go to the LAVATORY!!!" I just use the British 'loo'.

Both of the Harry Potter films refer to the "girls' bathroom" when they mean the girls' lavatory/toilet/loo. It shows the scriptwriter is American and it really jars.

The fact is in most American houses the toilet, sink, and bath tub are all in the same room. Americans like to call this room the bathroom...

To complicate matters we have real estate terms such as a full bath which contains a commode, sink, and tub, a 3/4 bath containing a shower, sink and commode, and a 1/2 bath, which has only a commode and sink.
http://realtytimes.com/rtcpages/19990202_mlscode.htm

McGruff?
I can't believe you refer to a toilet as a commode ?

I refer to the actual fixture as a toilet and wasn't aware there was anything rude about it. I don't often hear people say "I'm going to the toilet," it is usually bathroom. Restroom is usually used when referring to public facilities. We used commode in drafting.

I think I understand what you mean by "drafting" McGruff, but this is an Americanism and not perhaps understood by most Brits.
Could you explain what you mean by it ?

Drafting is mechanical drawing such as blueprints and house plans.

"Can" and "john" are slang to me. Neither is particularly rude, but I wouldn't use them when talking to a stranger, I'd ask for the restroom, bathroom or washroom. Oops, there's another word for it!

"John" usually refers to a port-a-pot or johnny-on-the-spot, those stinky one-holers set up at outdoor events. "One-holer" is a country term for an outhouse, two-holer being the delux edition. I've also seen a family-sized three-holer.

Bryce is totally right!
Saying toilet is rude in a formal situation! It really depends on whom you're talking to I guess.
But I always go with the rule:
US: bathroom in a house, restroom in public
other English places: washroom

Don't ask for 'washroom' in the UK unless you want to wash. This is a term used in industrial situations mostly - sometimes for a place where things like milk bottles get washed, or where mild decontamination is carried out.

In very informal situations (with close friends, etc) bog is fairly common (but not if your close friends are snobbish, posh or royal). Main informal word is loo, but toilet is quiet acceptable. Toilet is acceptable formally, too - but as with loo should be spoken quietly. Users of 'bog' generally don't worry about sound level. WC and Toilet are standard labelling on public facilities.

Restrooms are usually tents at big shows, often provided (with gentle propaganda) by church groups. They are for resting your feet, and possibly getting a cup of tea. They do not have loos. Or baths. UK bathrooms may or may not contain a loo, but always have a bath or shower.

(Source: Baloo travels around a lot and therefore has an interest in obtaining relief in assorted locations where public facilities appear to be few. He also maintains a mental list of safe bushes for emergency use...)

In Ireland, the preferred slang term is "the jacks" and isn't considered rude. Curious, since Jack is a slang name for John (or was -- seems to be a proper name now). As a kid watching imported American TV shows, I had the vague impression that American "restrooms" had a toilet, sink, and couch.