Well, it could be that it's "coincidence" - but I would like to think it is more related to one of two things, or possibly both.
A) The dinged up one of the sensors when they were bleeding the system at each wheel.
B) They just didn't get the system bled properly or messed with the accumulator pressures on the ABS unit.
I'm going for a bad or improper bleed. OR, could be that the sensor just really did fail.
Remember though, once the fail-safe relay detects a problem or failure within the main system and activates the ABS light, your ABS is no longer functioning. Your brakes start functioning as they would in any conventional brake system.
So keep that in mind in inclement weather and other emergency situations where you might suddenly think you can depend on your ABS to keep you out of trouble ... because with that light on, it's not going to happen that way.
Oh and one other thing -- if you do let them replace the sensor, make sure you tell them up front that you would like to see the bad sensor while still on the car and that you want it given to you once they remove it. Mark it with a permanent marker so you have proof they indeed did do the work they told you they were going to do.
Hey, it is YOUR car and it is YOUR money. It might piss someone off in the shop, but so what? Tell them that you have to do what you have to do, money is tight and you're just making sure people are keeping it honest.
Something I have done many times as far as marking the part, and I definitely ALWAYS ask for my replaced or repaired parts to be returned to me if I ever have work done on my car by anyone other than my regular mechanic.
Best of luck, and make sure you come back by and let us know what the final outcome is.
** I've included shop manual images (below) of the front and rear sensor locations, as well as an actual sensor set, showing the full cabling, wiring, etc. Just so you can go into this semi-prepared, if you chose to.
CLICK ON IMAGE TO ENLARGE
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