My father did the same thing for a long time. He was a non-practicing Catholic who occasionally attended and received holy communion, and he also attended an evangelical Church, receiving communion when it was offered. He didn't know any better. He was one of those people that didn't take Church very seriously, so in the long run, for him, it didn't matter at all.
I think that this has a lot to do with parishioners of all denominations being ignorant about what Churches are "closed communion". I have never, ever seen a Catholic Church refuse holy communion to anyone. When I was about 11, I went to a RC Church for a long time. I was officially Presbyterian, but I received communion there every Sunday. Neither I, nor the priest, thought anything of it. In my young mind, I was a Christian, so I was fine receiving the body and blood of Christ anywhere, as he commanded me to do in the Bible.
I think that another contributing factor is the demographics of RC communities. About half of the people in my community are RC, roughly one quarter are traditional Protestants (Anglicans, Lutherans, Methodists, etc.) The other 1/4 is Evangelical.
We have about seven Catholic Churches in my community. So many people go to one Church at any given Sunday, and the crowds tend to vary from week to week. For a priest to know everyone and whether they're "approved" for holy communion is nearly impossible. It's up to the people to judge whether they are "worthy". That's the way it is in my community, anyway.