'They ride a bike.' vs 'They ride bikes.'
In English, generically engaging in an activity can be given in the singular form.
- play a musical instrument
- ride a bike
- mow a lawn
- cook a meal
- drive a car
- crank out an article [write an article]
- take a taxi
- make a profit
It can be argued that all the examples above link the verb to the item to express engaging in an activity. The nouns are "intimately" related to the activity.
- Many people play a musical instrument.
- Most children here learn to ride a bike.
- Mowing a lawn is not something I enjoy.
- He likes to cook a meal from time to time.
They usually ride a bike to that location. [a general activity]
Compare:
They usually hitch a ride to that location. [a general activity]
They ride company bikes, not rental ones. [the plural is not about the activity per se, it is about the type of thing used for it.]
And to show the question:
They ride bikes. They don't ride horses. [bikes is contrasted with horses]
They ride a bike [when they feel like it.] [the activity]
When the emphasis in the sentence is on a general activity, the verb + a + item can be used. When the emphasis is not on the activity but on the item, the plural is used.
That man likes to make a profit. [general] That man likes to make profits. [specific]
proof of this pudding :)
In sum: the difference, therefore, is between a general idea versus a specific idea.