Simon belonged to a movement called the Zealots, who formed in the early first century AD with the aim of freeing Israel from Roman occupation. The Zealots were key players in the Jewish Revolt of AD 66 which led to the Roman destruction of the Temple and much of Jerusalem in AD 70.
Apparently, Zealots were known to assassinate people like tax collectors for their collaboration with the hated Roman occupiers. So it's likely that Simon would have viewed Matthew in a very dim light. From Boyd's book:
Historical records indicate that the zealots despised tax collectors even more than they despised the Romans, for tax collectors not only paid taxes to support the Roman government (something zealots deplored), but they actually made their living collecting taxes from other Jews on Rome's behalf. Even worse, tax collectors often enhanced their income by charging more than was due and keeping the difference.
It must be the case, then, that being disciples of Jesus gave Matthew and Simon something in common that was so strong it over-rode their (pretty major) political differences. They spent three years or so on mission with Jesus, learning from and ministering with him. No doubt, this would have changed their political views, so that at least (Boyd again) 'the tax collector would no longer cheat his clients and the zealot no longer kill his opponents'. It reminded me of Isaiah 11, which imagines a time when 'the wolf and the lamb will live together' and 'the baby will play safely near the hole of a cobra'.
Oddly, though, we have no record of Jesus directly addressing either of the political views that Simon and Matthew brought along when Jesus invited them to follow him. Jesus teaches from an entirely different perspective and refuses to get drawn in to passing comment on which political view or 'kingdom of the world' is better. The kingdom of God is simply of a completely different nature. I'll finish with Greg Boyd's summary of the issue, which I guess applies more to what he sees in his country (the USA) but has certainly given me food for thought:
We have lost the simplicity of the kingdom of God and have largely forsaken the difficult challenge of living out the kingdom. We have forgotten, if ever we were taught, the simple principle that the kingdom of God looks like Jesus and that our sole task as kingdom people is to mimic the love he revealed on Calvary. Our unique calling as kingdom people is not to come up with God's opinion of the right solution to political issues. Our unique calling is simply to replicate Christ's sacrificial love in service to the world.