Are single chainring mountain bikes usual? I just bought my son a typical+ mountain bike in a typical Frech sports shop (a BTWIN 530 at Décathlon) and realized that it has only a single sprocket at the front, and 9 sprockets at the back:

I have never seen that before (not that I looked very much, but I am used to the 3 + ~7 combinations) and was wondering whether this is

usual
expected
unusual but why not
you have been scammed

I can, worst case, return the bike but Decathlon, while being a Mr and Mrs Everyone shop, is usually fine (I casually rode their bikes for years).
LATE EDIT: thank you for all the answers so far - in light of the excellent details I got there I should have probably noted that this is a bike for casual, usually on-road biking.
LAST EDIT: thank you for all the answers - this is really a great SE site. It was near to impossible to choose THE answer, sorry for the others where I could just +1 (the update on road usage was great, thank you)
A 1x drivetrain (single chainring) is pretty much the standard now in mountain biking - you will notice the rear cassette is much wider than the classic 3x7 setup you are used to.  The wide rear cassette gives a wide range of gears (although not always as wide as the old 3x7). Most also have also moved to more speeds(sprockets) in the rear (i.e., 11-12 speed), if the 1x9 has a large range (which it appears that it does), it will have larger steps between gears than a 3x7 drivetrain you are used to.
Other changes in modern mountain bike drive trains are:

rear derailleurs are usually clutched to better retain the chain and control chain slap
a front single chain rings using narrow/wide tooth configuration to again better retain the chain

Better chain retention and a single rear derailleur means that riders have to think less about gears. In the modern setup you do not have to think about chain management, as the chains generally remain in the selected gear. Contrast this against a 3x7, where you often had to remember to shift Big-big (big ring, big cassette) before downhills to improve chain retention. This leaves the rider to focus more on navigating the terrain.
In terms of the pictured bike, the drawbacks relative to a 3x7 are likely a reduced range between the hardest and easiest gear and larger jumps between gears.

Update  - on road usage
The OP updated the question to note that the bike is intended for casual on road use.  The 1x setup is really tune  for trail usage were speeds are slower. Relative to the reference drive train (a 3x7), out of the box it will be less functional for on road use due to it being under-geared for the road (i.e., on the road you may find there is not a &quot;hard&quot; enough gear).  This can be alleviated by swapping out the front chain ring for a larger chain ring (as well as road friendly tires).  The gearing is more &quot;tunable&quot; than  reference drive train, where there were little option in the market  to change the 3x chainrings to different sizes.