Colons need to be escaped

$ psql -h vdb.engineering.redhat.com -p 35432 -U sgreen -d EngVDBR
Password for user sgreen:
psql (9.1.4, server 8.1.4)
WARNING: psql version 9.1, server version 8.1.
Some psql features might not work.
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.

EngVDBR=> SELECT class, COUNT FROM Bugzilla.audit_log GROUP BY class;
class | count
--------------------------------------------------+------
Bugzilla::Component | 3686
Bugzilla::Extension::ExternalBugs::Bug | 3110
Bugzilla::Extension::ExternalBugs::Type | 4
Bugzilla::Extension::ExternalBugs::Type::Bugzilla | 1
Bugzilla::Extension::Releases::Release | 5
Bugzilla::Field | 27
Bugzilla::Field::Choice::cf_extra_component | 421
Bugzilla::Field::Choice::cf_extra_version | 20
Bugzilla::Field::Choice::cf_verified | 1
Bugzilla::FlagType | 89
Bugzilla::Group | 34
Bugzilla::Milestone | 81
Bugzilla::Product | 30
Bugzilla::Release | 66
Bugzilla::User | 3316
Bugzilla::Version | 79
(16 rows)

EngVDBR=> SELECT * FROM Bugzilla.audit_log WHERE class = 'Bugzilla::Component' LIMIT 1;
user_id | class | object_id | field | removed | added | at_time
-----------------------------------------------
(0 rows)

EngVDBR=> SELECT * FROM Bugzilla.audit_log WHERE class = 'Bugzilla\:\:Component' LIMIT 1;
user_id | class | object_id | field | removed | added |
at_time
--------------------------------------------------------------
---------------------
271726 | Bugzilla::Component | 91289 | initialowner | 173606 | 295575 | 2
012-05-19 19:33:15.0
(1 row)

Question is. Why do the colons need to be backspaced to get a result?