I sure know a lot of it happens where I live (AU). The agencies just want to build up a library of people they can call on when, and if, they get a suitable request.
And the reverse is the same, every time I advertised a position, I'd get a huge number of calls from people asking about the position, keep me on the phone for 20 minutes and then say "my agency has just the right person for that role, let us do your recruiting ..."
I even took to adding "No Agencies Please" but still happened.
I suggest you be careful about ads that appear very general in their requirements, a good position ad is quite specific about what they want you to do, you would normally 'adjust' your resume to focus on that job. The agencies are just sweeping up everything they can find and it shows.
On the other hand, if work is slow, having your resume with every agency in town may not be a bad thing, just don't assume its a good thing ! If they want to charge you a fee to have you on file, walk away.
Davo