What Beeper is doing is not illegal, they are not breaking any laws, all they have done is used Apple's services in a way Apple does not like so Apple has shut it down.
As pointed out in
@Sorinut post above, everything Beeper has done is within the acceptable confines of the law BUT Apple used the excuse 'Security and Privacy' to shut them down. THIS is the probable reason why lawmakers are looking into this because whether Apple likes it or not, not in this case, Beeper has done nothing wrong, again as per
@Sorinut's post BUT Apple used 'Privacy and Security' as the reason to shut them down. That action can be construed as anticompetitive because Apple is preventing a competitor from doing something that is perfectly acceptable within the confines of the law.