I was able to confirm the reported behaviour. 

1.20.14 honours  '--contraints tag=...' and errors when the request can not be fulfilled. 

$ juju --show-log bootstrap -e maas-env2-weekly --constraints tags=medium
2015-01-15 00:31:38 INFO juju.cmd supercommand.go:37 running juju [1.20.14-trusty-amd64 gc]
2015-01-15 00:31:43 INFO juju.environs.bootstrap bootstrap.go:48 bootstrapping environment "maas-env2-weekly"
2015-01-15 00:31:43 INFO juju.environs.tools tools.go:187 filtering tools by released version
2015-01-15 00:31:43 INFO juju.environs.tools tools.go:87 reading tools with major.minor version 1.20
2015-01-15 00:31:43 INFO juju.environs.tools tools.go:98 filtering tools by series: trusty
2015-01-15 00:31:43 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:31:43 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:31:43 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:31:43 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:31:43 INFO juju.environs.bootstrap bootstrap.go:60 newest version: 1.20.14
2015-01-15 00:31:44 INFO juju.environs.bootstrap bootstrap.go:88 picked bootstrap tools version: 1.20.14
Launching instance
2015-01-15 00:31:50 ERROR juju.provider.common bootstrap.go:136 bootstrap failed: cannot start bootstrap instance: cannot run instances: gomaasapi: got error back from server: 409 CONFLICT (No available node matches constraints: tags=medium)
Bootstrap failed, destroying environment
2015-01-15 00:31:50 INFO juju.provider.common destroy.go:15 destroying environment "maas-env2-weekly"
2015-01-15 00:31:50 ERROR juju.cmd supercommand.go:323 cannot start bootstrap instance: cannot run instances: gomaasapi: got error back from server: 409 CONFLICT (No available node matches constraints: tags=medium)

1.21-beta4 does not:
juju --show-log bootstrap -e maas-env2-weekly --constraints tags=medium
2015-01-15 00:34:35 INFO juju.cmd supercommand.go:37 running juju [1.21-beta4-trusty-amd64 gc]
2015-01-15 00:34:35 WARNING juju.environs config.go:127 Config attribute "tools-metadata-url" (https://swift.canonistack.canonical.com/v1/AUTH_526ad877f3e3464589dc1145dfeaac60/juju-dist/weekly/tools) is deprecated. 
It is replaced by "agent-metadata-url" attribute. 
Your configuration should be updated to set "agent-metadata-url" as follows 
agent-metadata-url: https://swift.canonistack.canonical.com/v1/AUTH_526ad877f3e3464589dc1145dfeaac60/juju-dist/weekly/tools.
2015-01-15 00:34:35 INFO juju.network network.go:106 setting prefer-ipv6 to false
Bootstrapping environment "maas-env2-weekly"
2015-01-15 00:34:40 INFO juju.environs.tools tools.go:86 reading tools with major.minor version 1.21
2015-01-15 00:34:40 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:34:40 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:34:40 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:34:40 INFO juju.utils http.go:59 hostname SSL verification enabled
2015-01-15 00:34:40 INFO juju.utils http.go:59 hostname SSL verification enabled
Starting new instance for initial state server
Launching instance
.
.
.